xtensa: Implement _xt_ints_on/off for asm2

Legacy xtensa had a rather complicated implementation of en/disabling
interrupts, owing to the "software priority" feature (which plays
games with INTENABLE and INTLEVEL to allow for interrupts to interrupt
each other outside their normal priorities).  But that's not a Zephyr
feature, it's enabled by a XT_USE_SWPRI value that comes from platform
headers and isn't enabled on any of our boards.  Dead code, basically.

Replace with the obvious implementation when asm2 is in use.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>