target/sparc: Fix RETURN

target/sparc: Fix RETURN

Perform window restore before pc update. Required in order
to recognize any window underflow trap with the current pc.

Fixes: 86b82fe021f4 ("target/sparc: Move JMPL, RETT, RETURN to decodetree")
Reported-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Tested-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Acked-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>