target-xtensa: record available window in TB flags

Record last valid 4-register window pane number in TB flags so that a
window overflow exception throw point is known at the translation time.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
target-xtensa: don't generate dead code

Don't generate TCG operations when privilege, register window or
coprocessor checks fail.

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