target-xtensa: implement windowed registers

See ISA, 4.7.1 for details.

Physical registers and currently visible window are separate fields in
CPUEnv. Only current window is accessible to TCG. On operations that
change window base helpers copy current window to and from physical
registers.

Window overflow check described in 4.7.1.3 is in separate patch.

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