Fix _sbrk() implementation for Cortex-M parts.
Fix _sbrk() implementation for Cortex-M parts.

A recent commit, 43acaa4166, to get _sbrk() to build successfully for
LPC2368 broke the Cortex-M implementation.  __get_MSP() isn't ever
defined as a macro, it is an inline function.  This means that the
code would always be compiled to use SP instead of MSP on Cortex-M
parts.  I switched the code to instead use the TARGET_ARM7 define
to choose which stack pointer to utilize.

I tested this fix by making sure that the LPC2368 version of the mbed
SDK would still build successfully with the Python scripts and that the
NET1 test still built and ran successfully on my mbed-LPC1768 device.

DanKupiniak/mbed,devanlai/mbed,nvlsianpu/mbed,larks/mbed,masaohamanaka/mbed,betzw/mbed-os,Tiryoh/mbed,maximmbed/mbed,mbedmicro/mbed,GustavWi/mbed,svastm/mbed,mbedmicro/mbed,ryankurte/mbed-os,naves-thiago/mbed-midi,mazimkhan/mbed-os,autopulated/mbed,fpiot/mbed-ats,nabilbendafi/mbed,mikaleppanen/mbed-os,FranklyDev/mbed,bcostm/mbed-os,bentwire/mbed,JasonHow44/mbed,CalSol/mbed,struempelix/mbed,kpurusho/mbed,theotherjimmy/mbed,bremoran/mbed-drivers,jferreir/mbed,karsev/mbed-os,cvtsi2sd/mbed-os,c1728p9/mbed-os,jpbrucker/mbed,RonEld/mbed,theotherjimmy/mbed,dbestm/mbed,fahhem/mbed-os,karsev/mbed-os,jpbrucker/mbed,andreaslarssonublox/mbed,RonEld/mbed,al177/mbed,fanghuaqi/mbed,geky/mbed,arostm/mbed-os,mnlipp/mbed,nRFMesh/mbed-os,bulislaw/mbed-os,tung7970/mbed-os-1,JasonHow44/mbed,K4zuki/mbed,radhika-raghavendran/mbed-os5.1-onsemi,NitinBhaskar/mbed,NXPmicro/mbed,Willem23/mbed,c1728p9/mbed-os,bcostm/mbed-os,pbrook/mbed,bentwire/mbed,RonEld/mbed,mazimkhan/mbed-os,Sweet-Peas/mbed,tung7970/mbed-os,ARM-software/mbed-beetle,ban4jp/mbed,Willem23/mbed,dbestm/mbed,Marcomissyou/mbed,Tiryoh/mbed,adustm/mbed,al177/mbed,nabilbendafi/mbed,karsev/mbed-os,pedromes/mbed,pedromes/mbed,rgrover/mbed,bcostm/mbed-os,kjbracey-arm/mbed,mmorenobarm/mbed-os,c1728p9/mbed-os,fvincenzo/mbed-os,radhika-raghavendran/mbed-os5.1-onsemi,wodji/mbed,YarivCol/mbed-os,adustm/mbed,NordicSemiconductor/mbed,j-greffe/mbed-os,wodji/mbed,bentwire/mbed,JasonHow44/mbed,bremoran/mbed-drivers,bikeNomad/mbed,Shengliang/mbed,DanKupiniak/mbed,HeadsUpDisplayInc/mbed,svogl/mbed-os,wodji/mbed,xcrespo/mbed,infinnovation/mbed-os,mbedmicro/mbed,sam-geek/mbed,Timmmm/mbed,bulislaw/mbed-os,masaohamanaka/mbed,Archcady/mbed-os,sg-/mbed-drivers,pi19404/mbed,maximmbed/mbed,catiedev/mbed-os,devanlai/mbed,jrjang/mbed,andcor02/mbed-os,fvincenzo/mbed-os,fanghuaqi/mbed,adustm/mbed,CalSol/mbed,bentwire/mbed,DanKupiniak/mbed,bcostm/mbed-os,HeadsUpDisplayInc/mbed,al177/mbed,Willem23/mbed,naves-thiago/mbed-midi,c1728p9/mbed-os,EmuxEvans/mbed,bentwire/mbed,larks/mbed,nvlsianpu/mbed,Shengliang/mbed,struempelix/mbed,YarivCol/mbed-os,pedromes/mbed,jamesadevine/mbed,jpbrucker/mbed,radhika-raghavendran/mbed-os5.1-onsemi,jamesadevine/mbed,jrjang/mbed,ban4jp/mbed,al177/mbed,pradeep-gr/mbed-os5-onsemi,bikeNomad/mbed,pi19404/mbed,Shengliang/mbed,logost/mbed,fahhem/mbed-os,andcor02/mbed-os,Shengliang/mbed,svastm/mbed,adustm/mbed,ban4jp/mbed,larks/mbed,j-greffe/mbed-os,getopenmono/mbed,Marcomissyou/mbed,naves-thiago/mbed-midi,catiedev/mbed-os,jamesadevine/mbed,nvlsianpu/mbed,jrjang/mbed,NitinBhaskar/mbed,jferreir/mbed,monkiineko/mbed-os,svogl/mbed-os,cvtsi2sd/mbed-os,alertby/mbed,mikaleppanen/mbed-os,mnlipp/mbed,mazimkhan/mbed-os,svastm/mbed,NXPmicro/mbed,jeremybrodt/mbed,jrjang/mbed,kpurusho/mbed,svogl/mbed-os,logost/mbed,Willem23/mbed,j-greffe/mbed-os,devanlai/mbed,pbrook/mbed,hwfwgrp/mbed,K4zuki/mbed,kpurusho/mbed,hwfwgrp/mbed,struempelix/mbed,hwfwgrp/mbed,tung7970/mbed-os-1,getopenmono/mbed,sam-geek/mbed,catiedev/mbed-os,Tiryoh/mbed,tung7970/mbed-os-1,nabilbendafi/mbed,screamerbg/mbed,jeremybrodt/mbed,K4zuki/mbed,fanghuaqi/mbed,NitinBhaskar/mbed,maximmbed/mbed,dbestm/mbed,ban4jp/mbed,NXPmicro/mbed,ARM-software/mbed-beetle,masaohamanaka/mbed,monkiineko/mbed-os,karsev/mbed-os,Sweet-Peas/mbed,Shengliang/mbed,Tiryoh/mbed,kl-cruz/mbed-os,EmuxEvans/mbed,nvlsianpu/mbed,al177/mbed,j-greffe/mbed-os,svastm/mbed,masaohamanaka/mbed,dbestm/mbed,Archcady/mbed-os,netzimme/mbed-os,jferreir/mbed,theotherjimmy/mbed,kl-cruz/mbed-os,kjbracey-arm/mbed,mazimkhan/mbed-os,c1728p9/mbed-os,bentwire/mbed,kpurusho/mbed,Archcady/mbed-os,tung7970/mbed-os,nabilbendafi/mbed,adamgreen/mbed,GustavWi/mbed,EmuxEvans/mbed,mikaleppanen/mbed-os,GustavWi/mbed,rosterloh/mbed,alertby/mbed,logost/mbed,YarivCol/mbed-os,rosterloh/mbed,pradeep-gr/mbed-os5-onsemi,mmorenobarm/mbed-os,mmorenobarm/mbed-os,NordicSemiconductor/mbed,monkiineko/mbed-os,adamgreen/mbed,bulislaw/mbed-os,mnlipp/mbed,screamerbg/mbed,struempelix/mbed,andcor02/mbed-os,NXPmicro/mbed,brstew/MBED-BUILD,brstew/MBED-BUILD,jpbrucker/mbed,Archcady/mbed-os,ryankurte/mbed-os,infinnovation/mbed-os,pedromes/mbed,HeadsUpDisplayInc/mbed,andcor02/mbed-os,nRFMesh/mbed-os,fpiot/mbed-ats,kl-cruz/mbed-os,Archcady/mbed-os,wodji/mbed,Sweet-Peas/mbed,sam-geek/mbed,tung7970/mbed-os,dbestm/mbed,sam-geek/mbed,JasonHow44/mbed,masaohamanaka/mbed,betzw/mbed-os,logost/mbed,nabilbendafi/mbed,jferreir/mbed,jeremybrodt/mbed,ryankurte/mbed-os,rgrover/mbed,jrjang/mbed,NitinBhaskar/mbed,masaohamanaka/mbed,monkiineko/mbed-os,arostm/mbed-os,theotherjimmy/mbed,radhika-raghavendran/mbed-os5.1-onsemi,screamerbg/mbed,brstew/MBED-BUILD,andcor02/mbed-os,RonEld/mbed,K4zuki/mbed,infinnovation/mbed-os,cvtsi2sd/mbed-os,YarivCol/mbed-os,jpbrucker/mbed,Marcomissyou/mbed,bikeNomad/mbed,kpurusho/mbed,Shengliang/mbed,EmuxEvans/mbed,rosterloh/mbed,andreaslarssonublox/mbed,adamgreen/mbed,mazimkhan/mbed-os,devanlai/mbed,jferreir/mbed,rosterloh/mbed,adamgreen/mbed,nvlsianpu/mbed,rgrover/mbed,ARM-software/mbed-beetle,RonEld/mbed,fanghuaqi/mbed,pedromes/mbed,logost/mbed,Marcomissyou/mbed,bulislaw/mbed-os,tung7970/mbed-os,jamesadevine/mbed,infinnovation/mbed-os,YarivCol/mbed-os,svastm/mbed,JasonHow44/mbed,ban4jp/mbed,GustavWi/mbed,RonEld/mbed,kpurusho/mbed,hwfwgrp/mbed,YarivCol/mbed-os,nRFMesh/mbed-os,NordicSemiconductor/mbed,netzimme/mbed-os,pbrook/mbed,FranklyDev/mbed,maximmbed/mbed,Timmmm/mbed,xcrespo/mbed,tung7970/mbed-os-1,geky/mbed,mbedmicro/mbed,iriark01/mbed-drivers,jpbrucker/mbed,CalSol/mbed,FranklyDev/mbed,mbedmicro/mbed,monkiineko/mbed-os,devanlai/mbed,geky/mbed,autopulated/mbed,getopenmono/mbed,betzw/mbed-os,alertby/mbed,cvtsi2sd/mbed-os,Sweet-Peas/mbed,radhika-raghavendran/mbed-os5.1-onsemi,netzimme/mbed-os,betzw/mbed-os,pradeep-gr/mbed-os5-onsemi,netzimme/mbed-os,EmuxEvans/mbed,fpiot/mbed-ats,larks/mbed,mmorenobarm/mbed-os,rosterloh/mbed,CalSol/mbed,mikaleppanen/mbed-os,catiedev/mbed-os,mikaleppanen/mbed-os,cvtsi2sd/mbed-os,pi19404/mbed,xcrespo/mbed,ban4jp/mbed,devanlai/mbed,xcrespo/mbed,Sweet-Peas/mbed,JasonHow44/mbed,jamesadevine/mbed,netzimme/mbed-os,pbrook/mbed,infinnovation/mbed-os,fpiot/mbed-ats,GustavWi/mbed,adamgreen/mbed,pbrook/mbed,j-greffe/mbed-os,screamerbg/mbed,fpiot/mbed-ats,radhika-raghavendran/mbed-os5.1-onsemi,betzw/mbed-os,tung7970/mbed-os-1,K4zuki/mbed,alertby/mbed,Timmmm/mbed,struempelix/mbed,andreaslarssonublox/mbed,jeremybrodt/mbed,andcor02/mbed-os,pradeep-gr/mbed-os5-onsemi,fvincenzo/mbed-os,brstew/MBED-BUILD,NXPmicro/mbed,bulislaw/mbed-os,pi19404/mbed,screamerbg/mbed,hwfwgrp/mbed,alertby/mbed,ryankurte/mbed-os,iriark01/mbed-drivers,adustm/mbed,bikeNomad/mbed,svogl/mbed-os,kl-cruz/mbed-os,rosterloh/mbed,adustm/mbed,dbestm/mbed,catiedev/mbed-os,autopulated/mbed,bcostm/mbed-os,nRFMesh/mbed-os,larks/mbed,bikeNomad/mbed,karsev/mbed-os,bcostm/mbed-os,xcrespo/mbed,cvtsi2sd/mbed-os,logost/mbed,screamerbg/mbed,alertby/mbed,FranklyDev/mbed,pbrook/mbed,fvincenzo/mbed-os,jrjang/mbed,fahhem/mbed-os,autopulated/mbed,kjbracey-arm/mbed,mazimkhan/mbed-os,fahhem/mbed-os,NordicSemiconductor/mbed,sam-geek/mbed,xcrespo/mbed,catiedev/mbed-os,andreaslarssonublox/mbed,fpiot/mbed-ats,CalSol/mbed,HeadsUpDisplayInc/mbed,mmorenobarm/mbed-os,sg-/mbed-drivers,infinnovation/mbed-os,fahhem/mbed-os,EmuxEvans/mbed,pedromes/mbed,K4zuki/mbed,geky/mbed,0xc0170/mbed-drivers,DanKupiniak/mbed,geky/mbed,mnlipp/mbed,maximmbed/mbed,nRFMesh/mbed-os,arostm/mbed-os,NitinBhaskar/mbed,pi19404/mbed,wodji/mbed,ARM-software/mbed-beetle,arostm/mbed-os,betzw/mbed-os,Timmmm/mbed,getopenmono/mbed,monkiineko/mbed-os,pi19404/mbed,nvlsianpu/mbed,kl-cruz/mbed-os,Archcady/mbed-os,ryankurte/mbed-os,Sweet-Peas/mbed,jamesadevine/mbed,j-greffe/mbed-os,al177/mbed,Marcomissyou/mbed,fanghuaqi/mbed,arostm/mbed-os,getopenmono/mbed,fahhem/mbed-os,mmorenobarm/mbed-os,netzimme/mbed-os,svogl/mbed-os,wodji/mbed,pradeep-gr/mbed-os5-onsemi,maximmbed/mbed,Timmmm/mbed,jferreir/mbed,theotherjimmy/mbed,c1728p9/mbed-os,mnlipp/mbed,svogl/mbed-os,Timmmm/mbed,naves-thiago/mbed-midi,struempelix/mbed,getopenmono/mbed,nabilbendafi/mbed,kl-cruz/mbed-os,kjbracey-arm/mbed,karsev/mbed-os,hwfwgrp/mbed,brstew/MBED-BUILD,Marcomissyou/mbed,fvincenzo/mbed-os,autopulated/mbed,naves-thiago/mbed-midi,mnlipp/mbed,tung7970/mbed-os,NXPmicro/mbed,ryankurte/mbed-os,0xc0170/mbed-drivers,arostm/mbed-os,HeadsUpDisplayInc/mbed,rgrover/mbed,pradeep-gr/mbed-os5-onsemi,mikaleppanen/mbed-os,CalSol/mbed,naves-thiago/mbed-midi,bulislaw/mbed-os,larks/mbed,andreaslarssonublox/mbed,Willem23/mbed,HeadsUpDisplayInc/mbed,theotherjimmy/mbed,FranklyDev/mbed,rgrover/mbed,autopulated/mbed,Tiryoh/mbed,Tiryoh/mbed,brstew/MBED-BUILD,adamgreen/mbed,nRFMesh/mbed-os,jeremybrodt/mbed