target-sparc: fix register corruption in ldstub if there is no write permission

Signed-off-by: Artyom Tarasenko <atar4qemu@gmail.com>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
(cherry picked from commit b64d2e57)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>