tests: fatal: fix several issues

An errant commit accidentally disabled all testing of
hardware-based stack protection. Restore it, and work
around a problem with how these kinds of exceptions are
reported on ARM until #7706 is fixed.

We need to globally disable user mode due to how the
select statements in Kconfig work, the stack sentinel
is incompatible with user mode.

Some build warnings when compiling as native_posix
fixed.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>