arm: cortex_m: implement null pointer exception detection with MPU

Implementation for null pointer exception detection feature
using the MPU on Cortex-M. Null-pointer detection is implemented
by programming an MPU to guard a limited area starting at
address 0x0. on non ARMv8-M we program an MPU region with
No-access policy. On ARMv8-M we program a region with any
permissions, assuming the region will overlap with fixed
FLASH0 region. We add a compile-time message to warn the
user if the MPU-based null-pointer exception solution can
not be used (ARMv8-M only).

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