arm: cortex_m: fault: hook up debug monitor exception handler

Extend the debug monitor exception handler to
- return recoverable faults when the debug monitor
  is enabled but we do not get an expected DWT event,
- call a debug monitor routine to check for null pointer
  exceptions.

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