target-xtensa: implement RER/WER instructions

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
target-xtensa: add test_mmuhifi_c3 core

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
WIP: target-xtensa: add MX interrupt distributor

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