arch: arm: cortex_m: Convert cpu_idle from ASM to C

Asm is notoriously harder to maintain than C and requires core specific
adaptation which impairs even more the readability of the code.

This change reduces the need for core specific conditional compilation and
unifies irq locking code.

Signed-off-by: Wilfried Chauveau <wilfried.chauveau@arm.com>

# Conflicts:
#	soc/arm/nordic_nrf/nrf53/soc_cpu_idle.h