arm: cortex_m: Kconfig symbols for null pointer detection feature

Introduce the required Kconfig symbol framework for the
Cortex-M-specific null pointer dereferencing detection
feature. There are two implementations (based on DWT and
MPU) so we introduce the corresponding choice symbols,
including a choice symbol to signify that the feature
is to be disabled.

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