target-arm: A64: Avoid left shifting negative integers in disas_pc_rel_addr

Shifting a negative integer left is undefined behaviour in C.
Avoid it by assembling and shifting the offset fields as
unsigned values and then sign extending as the final action.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1423233250-15853-4-git-send-email-peter.maydell@linaro.org
