arch: arm: cortex_m: use C rather than asm in isr_wrapper & exc_exit

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

This is a first step in reducing the amount of ASM in arch/arm/cortex_m

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