tests/kernel/fatal: Add xtensa/asm2 to the "error returns" family
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 <c70f9a6bf6ee0cd69c8af4ce5e6b131945769315@intel.com>

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