cortex_m: re-add __start definition for cortex_m
cortex_m: re-add __start definition for cortex_m

This was removed as part of some Diab cleanup, however it is needed
and without the definition we get the following when building
for the FRDM-K64F  board:

arm-none-eabi-ld: warning: cannot find entry symbol __start; not setting start address

Change-Id: Ie0604a600b6f7a4faa321c58aa63c5617a163107
Signed-off-by: Anas Nashif <0d9952ec84ac43c159f6b7e7ed99a9080c00dd6e@intel.com>

explora26/zephyr,punitvara/zephyr,mbolivar/zephyr,rsalveti/zephyr,sharronliu/zephyr,bboozzoo/zephyr,runchip/zephyr-cc3200,mirzak/zephyr-os,aceofall/zephyr-iotos,bboozzoo/zephyr,erwango/zephyr,explora26/zephyr,fbsder/zephyr,fbsder/zephyr,GiulianoFranchetto/zephyr,fbsder/zephyr,aceofall/zephyr-iotos,erwango/zephyr,bboozzoo/zephyr,tidyjiang8/zephyr-doc,bigdinotech/zephyr,zephyriot/zephyr,bigdinotech/zephyr,kraj/zephyr,jamesonwilliams/zephyr-kernel,jamesonwilliams/zephyr-kernel,ldts/zephyr,holtmann/zephyr,runchip/zephyr-cc3200,ldts/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,punitvara/zephyr,mirzak/zephyr-os,jamesonwilliams/zephyr-kernel,ldts/zephyr,32bitmicro/zephyr,runchip/zephyr-cc3200,Vudentz/zephyr,explora26/zephyr,bboozzoo/zephyr,tidyjiang8/zephyr-doc,pklazy/zephyr,pklazy/zephyr,galak/zephyr,pklazy/zephyr,finikorg/zephyr,rsalveti/zephyr,galak/zephyr,aceofall/zephyr-iotos,rsalveti/zephyr,runchip/zephyr-cc3220,explora26/zephyr,bigdinotech/zephyr,mbolivar/zephyr,32bitmicro/zephyr,zephyriot/zephyr,kraj/zephyr,mirzak/zephyr-os,galak/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,erwango/zephyr,explora26/zephyr,32bitmicro/zephyr,tidyjiang8/zephyr-doc,coldnew/zephyr-project-fork,zephyrproject-rtos/zephyr,mbolivar/zephyr,bigdinotech/zephyr,holtmann/zephyr,holtmann/zephyr,coldnew/zephyr-project-fork,Vudentz/zephyr,holtmann/zephyr,mbolivar/zephyr,Vudentz/zephyr,galak/zephyr,ldts/zephyr,rsalveti/zephyr,holtmann/zephyr,sharronliu/zephyr,nashif/zephyr,nashif/zephyr,fractalclone/zephyr-riscv,kraj/zephyr,32bitmicro/zephyr,Vudentz/zephyr,ldts/zephyr,kraj/zephyr,nashif/zephyr,finikorg/zephyr,aceofall/zephyr-iotos,sharronliu/zephyr,coldnew/zephyr-project-fork,zephyrproject-rtos/zephyr,finikorg/zephyr,rsalveti/zephyr,runchip/zephyr-cc3200,runchip/zephyr-cc3220,GiulianoFranchetto/zephyr,tidyjiang8/zephyr-doc,zephyriot/zephyr,fbsder/zephyr,nashif/zephyr,erwango/zephyr,runchip/zephyr-cc3220,sharronliu/zephyr,GiulianoFranchetto/zephyr,fractalclone/zephyr-riscv,mbolivar/zephyr,fractalclone/zephyr-riscv,jamesonwilliams/zephyr-kernel,bboozzoo/zephyr,mirzak/zephyr-os,fractalclone/zephyr-riscv,Vudentz/zephyr,32bitmicro/zephyr,coldnew/zephyr-project-fork,zephyriot/zephyr,runchip/zephyr-cc3220,bigdinotech/zephyr,runchip/zephyr-cc3220,zephyrproject-rtos/zephyr,sharronliu/zephyr,GiulianoFranchetto/zephyr,tidyjiang8/zephyr-doc,kraj/zephyr,aceofall/zephyr-iotos,punitvara/zephyr,runchip/zephyr-cc3200,nashif/zephyr,erwango/zephyr,zephyriot/zephyr,jamesonwilliams/zephyr-kernel,coldnew/zephyr-project-fork,mirzak/zephyr-os,fbsder/zephyr,punitvara/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,pklazy/zephyr,punitvara/zephyr,pklazy/zephyr,fractalclone/zephyr-riscv