target-arm: A64: Avoid signed shifts in disas_ldst_pair()

Avoid shifting potentially negative signed offset values in
disas_ldst_pair() by keeping the offset in a uint64_t rather
than an int64_t.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1423233250-15853-5-git-send-email-peter.maydell@linaro.org