target/m68k: Perform writback before modifying SR

Writes to SR may change security state, which may involve
a swap of %ssp with %usp as reflected in %a7.  Finish the
writeback of %sp@+ before swapping stack pointers.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1206


Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Message-Id: <20220913142818.7802-3-richard.henderson@linaro.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>