arch: arm: cortex-m: introduce custom switch to main function

For the case of building Zephy with no-multithreading
support (CONFIG_MULTITHREADING=n) we introduce a
custom (ARCH-specific) function to switch to main()
from cstart(). This is required, since the Cortex-M
initialization code is temporarily using the interrupt
stack and main() should be using the z_main_stack,
instead. The function performs the PSP switching,
the PSPLIM setting (for ARMv8-M), FPU initialization
and static memory region initialization, to mimic
what the normal (CONFIG_MULTITHREADING=y) case does.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>