Create a platform specific file to override WEAK Handlers and pre-main hook mbed_sdk_init

I think this is an cleaner change than adding code to CMSIS files or creating a platform specific directory in /targets/cmsis. All other .o files are from /targets/cmsis except for retarget.o and that was the model for this change. Now each platform can create a platform_init.c file and use it to override default handlers or hook before main is called with mbed_sdk_init.

Tested with KL05Z, KL46Z, KL25Z and UBLOX_C027.