target-xtensa: implement RER/WER instructions

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
target-xtensa: add static vectors selection

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
target-xtensa: hack-fix gdbstub registers size

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>
target-xtensa: improve spin locking performance

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