target-xtensa: add extui unit test

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
target-xtensa: add fallthrough markers

Explicitly mark cases where we are deliberately falling through to the
following code.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
target-xtensa: refactor CCOUNT/CCOMPARE

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>