When building the test for Cortex-M, use an undefined
instruction to trigger a CPU fault, instead of null
pointer de-referencing. That's because null-pointer
access may, in TrustZone-enabled platforms, lead to
a system crash (due to security violation).
Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>