tests: z_except_reason() policy from usermode

User mode should be able to successfully induce a kernel
oops, or stack check fail fatal error. The latter is
required by compiler stack canaries.

User mode should not be able to induce a kernel panic, or
fake some other kind of exception.

Currently supported on ARM and x86 platforms.

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