arch: arm: cortex_m: fault: Improve handling of fault escalation on armv6-m

The current implementation causes a lockup of the core when the exception
originates from an invalid/unreachable pc. This fix first verifies on
armv6-m and armv8-m.base that pc was in an expected runnable region,
namely:

- .text
- .ramfunc
- .itcm

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