arch: arm: cortex_m: fix ASSERT expression in MemManage handler

We need to form the ASSERT expression inside the MemManage
fault handler for the case we building without USERSPACE
and STACK GUARD support, in the same way it is formed for
the case with USERSPACE or MPU STACK GUARD support, that
is, we only assert if we came across a stacking error.

Data access violations can still occur even without user
mode or guards, e.g. when trying to write to Read-only
memory (such as the code region).

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