tests/kernel/fatal: Add xtensa/asm2 to the "error returns" family

This test had to special case ARM, where error handlers are not
NORETURN functions.  The xtensa/asm2 layer has the same behavior
(albeit for a different reason).  Add it to the list, and clean up the
explanation a bit.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>