startup/core-clocks.cpp: Use .preinit_array to start early RTC.

Add a pointer to core_clocks_init_early to .preinit_array in order
to execute it automatically during early init.

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>
Experimental stack smashing protection runtime support.

Implemented the base functions necessary for GCC SSP (-fstack-protector) support.

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>