xtensa: stop execution under simulator for double exception

If running under Xtensa simulator, it is good to tell simulator
to stop execution once we reach double exception, as the current
double exception handler is simply an endless loop. If we turn
on tracing in the simulator, the output file would contain
an infinite iteration of this endless loop, and the simulator
needs to be stopped manually before the file size goes out of
control. So we need to tell the simulator to stop once
we reach this point instead of doing an endless loop.

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