arm: cortex-m: build debug.c for null-pointer detection feature

When we enable the null pointer exceptino feature (using DWT)
we include debug.c in the build. debug.c contains the functions
to configure and enable null pointer detection using the Data
Watchdog and Trace unit.

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