arch/arm: cortex_m: Disable DWT based null-pointer exception detection

Null-pointer exception detection using DWT is currently incompatible
with current openocd runner default implementation that leaves debug
mode on by default.
As a consequence, on all targets that use openocd runner, null-pointer
exception detection using DWT will generated an assert.
As a consequence, all tests are failing on such platforms.

Disable this until openocd behavior is fixed (#32984) and enable
the MPU based solution for now.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>