arch: xtensa: implement ARCH_EXCEPT

Triggers CPU exception with illegal instruction when z_except_reason
is called (e.g. in k_panic, k_oops). Creates exception stack frame
for use by coredump. Adds unique cause code for ARCH_EXCEPT. Disables
test case failure for qemu_xtensa.

Without an ARCH_EXCEPT implementation, z_except_reason calls
z_fatal_error directly with a null ESF and bypasses
xtensa_excint1_c's error logging. An ESF is required to coredump.

Signed-off-by: Lauren Murphy <lauren.murphy@intel.com>