arm: cortex_m: build time assert for null-pointer exception page size

We introduce build time asserts for
CONFIG_CORTEX_M_DEBUG_NULL_POINTER_EXCEPTION_PAGE_SIZE
to catch that the user-supplied value has, as requested
by the Kconfig symbol specification, a power of 2 value.
For the MPU-based implementation of null-pointer detection
we can use an existing macro for the build time assert,
since the region for catching null-pointer exceptions
is a regular MPU region, with different restrictions,
depending on the MPU architecture. For the DWT-based
implementation, we introduce a custom build-time assert.

We add also a run-time ASSERT for the MPU-based
implementation in ARMv8-M platforms, which require
that the null pointer exception detection page is
already mapped by the MPU.

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