Nordic: Fix multiple defined symbol.
Nordic: Fix multiple defined symbol.

The symbol Systick_Handler was defined as a weak symbol in us_ticker.c and
startup_nRF51822.s. While it work as expected when an application is compiled
from mbed OS source code it creates a duplicate symbol issue when mbed OS is
bundled as a library.

adamgreen/mbed,svogl/mbed-os,Archcady/mbed-os,HeadsUpDisplayInc/mbed,andcor02/mbed-os,fahhem/mbed-os,c1728p9/mbed-os,Archcady/mbed-os,mbedmicro/mbed,nRFMesh/mbed-os,Archcady/mbed-os,betzw/mbed-os,bcostm/mbed-os,mazimkhan/mbed-os,c1728p9/mbed-os,pradeep-gr/mbed-os5-onsemi,nRFMesh/mbed-os,kjbracey-arm/mbed,ryankurte/mbed-os,svogl/mbed-os,monkiineko/mbed-os,adamgreen/mbed,nRFMesh/mbed-os,infinnovation/mbed-os,catiedev/mbed-os,CalSol/mbed,monkiineko/mbed-os,YarivCol/mbed-os,CalSol/mbed,CalSol/mbed,monkiineko/mbed-os,fahhem/mbed-os,catiedev/mbed-os,svogl/mbed-os,betzw/mbed-os,adamgreen/mbed,monkiineko/mbed-os,ryankurte/mbed-os,pradeep-gr/mbed-os5-onsemi,catiedev/mbed-os,mazimkhan/mbed-os,svogl/mbed-os,kjbracey-arm/mbed,betzw/mbed-os,bcostm/mbed-os,YarivCol/mbed-os,NXPmicro/mbed,karsev/mbed-os,infinnovation/mbed-os,andcor02/mbed-os,andcor02/mbed-os,karsev/mbed-os,CalSol/mbed,NXPmicro/mbed,HeadsUpDisplayInc/mbed,HeadsUpDisplayInc/mbed,mbedmicro/mbed,Archcady/mbed-os,ryankurte/mbed-os,Archcady/mbed-os,infinnovation/mbed-os,ryankurte/mbed-os,mbedmicro/mbed,HeadsUpDisplayInc/mbed,infinnovation/mbed-os,mazimkhan/mbed-os,mazimkhan/mbed-os,ryankurte/mbed-os,fahhem/mbed-os,karsev/mbed-os,pradeep-gr/mbed-os5-onsemi,c1728p9/mbed-os,bcostm/mbed-os,infinnovation/mbed-os,catiedev/mbed-os,monkiineko/mbed-os,andcor02/mbed-os,CalSol/mbed,YarivCol/mbed-os,adamgreen/mbed,CalSol/mbed,pradeep-gr/mbed-os5-onsemi,svogl/mbed-os,bcostm/mbed-os,YarivCol/mbed-os,NXPmicro/mbed,ryankurte/mbed-os,mbedmicro/mbed,adamgreen/mbed,HeadsUpDisplayInc/mbed,nRFMesh/mbed-os,mbedmicro/mbed,karsev/mbed-os,YarivCol/mbed-os,NXPmicro/mbed,mazimkhan/mbed-os,bcostm/mbed-os,betzw/mbed-os,betzw/mbed-os,kjbracey-arm/mbed,nRFMesh/mbed-os,fahhem/mbed-os,fahhem/mbed-os,c1728p9/mbed-os,svogl/mbed-os,karsev/mbed-os,c1728p9/mbed-os,c1728p9/mbed-os,catiedev/mbed-os,andcor02/mbed-os,mazimkhan/mbed-os,YarivCol/mbed-os,bcostm/mbed-os,pradeep-gr/mbed-os5-onsemi,betzw/mbed-os,NXPmicro/mbed,andcor02/mbed-os,catiedev/mbed-os,HeadsUpDisplayInc/mbed,monkiineko/mbed-os,karsev/mbed-os,fahhem/mbed-os,Archcady/mbed-os,kjbracey-arm/mbed,NXPmicro/mbed,adamgreen/mbed,infinnovation/mbed-os,nRFMesh/mbed-os,pradeep-gr/mbed-os5-onsemi