arch: arm: cortex_m: Add debug.h header

Since #75677, it has not been possible to build with
CONFIG_NULL_POINTER_EXCEPTION_DETECTION_DWT enabled since there was no
declaration of z_arm_debug_enable_null_pointer_detection before its use
in arch/arm/core/cortex_m/prep_c.c.

This change creates an arch/arm/include/cortex_m/debug.h header that
contains declarations for the functions in
arch/arm/core/cortex_m/debug.c.

Signed-off-by: Tom Hughes <tomhughes@chromium.org>