xtensa: add support for TIF_NOTIFY_SIGNAL

Wire up TIF_NOTIFY_SIGNAL handling for xtensa.

Thanks to Max Filippov <jcmvbkbc@gmail.com> for making the asm correct.

Cc: linux-xtensa@linux-xtensa.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>