target/arm: Fix aa64 ldp register writeback

For "ldp x0, x1, [x0]", if the second load is on a second page and
the second page is unmapped, the exception would be raised with x0
already modified.  This means the instruction couldn't be restarted.

Cc: qemu-arm@nongnu.org
Cc: qemu-stable@nongnu.org
Reported-by: Andrew <andrew@fubar.geek.nz>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20170825224833.4463-1-richard.henderson@linaro.org
Fixes: https://bugs.launchpad.net/qemu/+bug/1713066


Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
[PMM: tweaked comment format]
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>