xtensa: only use BREAK if explicitly enabled

Introduce CONFIG_XTENSA_BREAK_ON_UNRECOVERABLE_EXCEPTIONS to
use BREAK instruction for unrecoverable exceptions. This
definitely requires debugger to be attached to the hardware
or simulator to catch that.

Also move the infinite loop to NOT result in an infinite
interrupt storm as the debug interrupt will be triggered over
and over again. Same for the simcall exit as it does not
need to be called repetitively.

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