xtensa: mmu: bail on semantic triple faults

There actually is no triple faults on Xtensa. Once PS.EXCM is
set, it keeps going through double exception vector for any
new exceptions. However, our exception code needs to unmask
PS.EXCM to enable register window operations. So after that,
any new exceptions will go through the kernel or user vectors
depending on PS.UM. If there is continuous faults, it may
keep ping-ponging between double and kernel/user exception
vectors that may never get resolved. Since we stash DEPC
during double exception, and the stashed one is only cleared
once the double exception has been processed, we can use
the stashed DEPC value to detect if the next exception could
be considered a triple fault. If such a case exists, simply
jump to an infinite loop, or quit the simulator, or invoke
debugger.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>