Fix GCC malloc lock
Fix GCC malloc lock

The malloc lock functions were not declared as extern "C" so they
are not getting linked to the standard library.  Add extern "C" to
fix this.

This bug was introduced in the commit:
d0b7b3b497ae1824bb8cb07603c90f5105d7818d -
Fix duplicate symbols for malloc lock and unlock

NXPmicro/mbed,nvlsianpu/mbed,andcor02/mbed-os,theotherjimmy/mbed,bulislaw/mbed-os,mikaleppanen/mbed-os,nRFMesh/mbed-os,pradeep-gr/mbed-os5-onsemi,mmorenobarm/mbed-os,fvincenzo/mbed-os,andreaslarssonublox/mbed,fanghuaqi/mbed,mbedmicro/mbed,adamgreen/mbed,maximmbed/mbed,kjbracey-arm/mbed,tung7970/mbed-os-1,CalSol/mbed,rgrover/mbed,nvlsianpu/mbed,fanghuaqi/mbed,catiedev/mbed-os,tung7970/mbed-os,svastm/mbed,svastm/mbed,DanKupiniak/mbed,fahhem/mbed-os,betzw/mbed-os,adamgreen/mbed,screamerbg/mbed,tung7970/mbed-os-1,adamgreen/mbed,NXPmicro/mbed,nvlsianpu/mbed,ryankurte/mbed-os,fahhem/mbed-os,netzimme/mbed-os,svastm/mbed,geky/mbed,CalSol/mbed,cvtsi2sd/mbed-os,catiedev/mbed-os,c1728p9/mbed-os,cvtsi2sd/mbed-os,monkiineko/mbed-os,monkiineko/mbed-os,karsev/mbed-os,Archcady/mbed-os,fahhem/mbed-os,svogl/mbed-os,infinnovation/mbed-os,mmorenobarm/mbed-os,mbedmicro/mbed,kl-cruz/mbed-os,arostm/mbed-os,rgrover/mbed,svogl/mbed-os,adustm/mbed,monkiineko/mbed-os,andcor02/mbed-os,DanKupiniak/mbed,infinnovation/mbed-os,tung7970/mbed-os,maximmbed/mbed,rgrover/mbed,bulislaw/mbed-os,mikaleppanen/mbed-os,mikaleppanen/mbed-os,fahhem/mbed-os,bcostm/mbed-os,c1728p9/mbed-os,catiedev/mbed-os,mazimkhan/mbed-os,screamerbg/mbed,tung7970/mbed-os,kl-cruz/mbed-os,cvtsi2sd/mbed-os,svogl/mbed-os,jeremybrodt/mbed,arostm/mbed-os,kl-cruz/mbed-os,fanghuaqi/mbed,nRFMesh/mbed-os,tung7970/mbed-os,NXPmicro/mbed,cvtsi2sd/mbed-os,pradeep-gr/mbed-os5-onsemi,j-greffe/mbed-os,monkiineko/mbed-os,DanKupiniak/mbed,nvlsianpu/mbed,adamgreen/mbed,radhika-raghavendran/mbed-os5.1-onsemi,adustm/mbed,j-greffe/mbed-os,CalSol/mbed,theotherjimmy/mbed,YarivCol/mbed-os,nRFMesh/mbed-os,andcor02/mbed-os,NXPmicro/mbed,netzimme/mbed-os,netzimme/mbed-os,j-greffe/mbed-os,RonEld/mbed,kl-cruz/mbed-os,fvincenzo/mbed-os,NXPmicro/mbed,fvincenzo/mbed-os,karsev/mbed-os,infinnovation/mbed-os,arostm/mbed-os,jeremybrodt/mbed,ARM-software/mbed-beetle,theotherjimmy/mbed,bcostm/mbed-os,fvincenzo/mbed-os,ryankurte/mbed-os,Archcady/mbed-os,catiedev/mbed-os,arostm/mbed-os,mmorenobarm/mbed-os,mazimkhan/mbed-os,HeadsUpDisplayInc/mbed,pradeep-gr/mbed-os5-onsemi,netzimme/mbed-os,theotherjimmy/mbed,pradeep-gr/mbed-os5-onsemi,karsev/mbed-os,andcor02/mbed-os,geky/mbed,screamerbg/mbed,svastm/mbed,nRFMesh/mbed-os,radhika-raghavendran/mbed-os5.1-onsemi,netzimme/mbed-os,mmorenobarm/mbed-os,andreaslarssonublox/mbed,nRFMesh/mbed-os,RonEld/mbed,mazimkhan/mbed-os,tung7970/mbed-os-1,YarivCol/mbed-os,nvlsianpu/mbed,tung7970/mbed-os-1,bulislaw/mbed-os,pradeep-gr/mbed-os5-onsemi,fanghuaqi/mbed,maximmbed/mbed,bcostm/mbed-os,bulislaw/mbed-os,CalSol/mbed,j-greffe/mbed-os,monkiineko/mbed-os,rgrover/mbed,j-greffe/mbed-os,mazimkhan/mbed-os,Archcady/mbed-os,svogl/mbed-os,screamerbg/mbed,screamerbg/mbed,YarivCol/mbed-os,mbedmicro/mbed,jeremybrodt/mbed,nRFMesh/mbed-os,kjbracey-arm/mbed,HeadsUpDisplayInc/mbed,adustm/mbed,andcor02/mbed-os,mazimkhan/mbed-os,Archcady/mbed-os,bcostm/mbed-os,RonEld/mbed,betzw/mbed-os,ARM-software/mbed-beetle,YarivCol/mbed-os,karsev/mbed-os,CalSol/mbed,geky/mbed,svastm/mbed,RonEld/mbed,ryankurte/mbed-os,Archcady/mbed-os,maximmbed/mbed,HeadsUpDisplayInc/mbed,mmorenobarm/mbed-os,ARM-software/mbed-beetle,ARM-software/mbed-beetle,NXPmicro/mbed,andreaslarssonublox/mbed,HeadsUpDisplayInc/mbed,mikaleppanen/mbed-os,catiedev/mbed-os,adustm/mbed,geky/mbed,tung7970/mbed-os-1,mmorenobarm/mbed-os,fahhem/mbed-os,andreaslarssonublox/mbed,Archcady/mbed-os,catiedev/mbed-os,adamgreen/mbed,karsev/mbed-os,rgrover/mbed,kjbracey-arm/mbed,geky/mbed,j-greffe/mbed-os,theotherjimmy/mbed,adustm/mbed,radhika-raghavendran/mbed-os5.1-onsemi,infinnovation/mbed-os,HeadsUpDisplayInc/mbed,arostm/mbed-os,andreaslarssonublox/mbed,ryankurte/mbed-os,infinnovation/mbed-os,mikaleppanen/mbed-os,bcostm/mbed-os,screamerbg/mbed,fahhem/mbed-os,arostm/mbed-os,nvlsianpu/mbed,jeremybrodt/mbed,c1728p9/mbed-os,kl-cruz/mbed-os,adamgreen/mbed,RonEld/mbed,jeremybrodt/mbed,netzimme/mbed-os,andcor02/mbed-os,ryankurte/mbed-os,theotherjimmy/mbed,svogl/mbed-os,pradeep-gr/mbed-os5-onsemi,DanKupiniak/mbed,betzw/mbed-os,c1728p9/mbed-os,betzw/mbed-os,bulislaw/mbed-os,betzw/mbed-os,c1728p9/mbed-os,adustm/mbed,fanghuaqi/mbed,infinnovation/mbed-os,radhika-raghavendran/mbed-os5.1-onsemi,kjbracey-arm/mbed,YarivCol/mbed-os,mbedmicro/mbed,fvincenzo/mbed-os,ryankurte/mbed-os,RonEld/mbed,YarivCol/mbed-os,maximmbed/mbed,c1728p9/mbed-os,svogl/mbed-os,cvtsi2sd/mbed-os,betzw/mbed-os,karsev/mbed-os,mazimkhan/mbed-os,bcostm/mbed-os,radhika-raghavendran/mbed-os5.1-onsemi,radhika-raghavendran/mbed-os5.1-onsemi,maximmbed/mbed,cvtsi2sd/mbed-os,kl-cruz/mbed-os,monkiineko/mbed-os,mikaleppanen/mbed-os,HeadsUpDisplayInc/mbed,tung7970/mbed-os,mbedmicro/mbed,bulislaw/mbed-os,CalSol/mbed