target-xtensa: implement accurate window check

See ISA, 4.7.1.3 for details.

Window check is inserted before commands that push "used register
watermark" beyond its current level. Used register watermark is reset on
instructions that change WINDOW_BASE/WINDOW_START SRs.

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