linux-user: Add signal handling for AArch64

linux-user: Add signal handling for AArch64

This patch adds signal handling for AArch64. The code is based on the
respective source in the Linux kernel.

Signed-off-by: Andreas Schwab <schwab@suse.de>
Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: John Rigby <john.rigby@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1378235544-22290-17-git-send-email-peter.maydell@linaro.org
Message-id: 1368505980-17151-10-git-send-email-john.rigby@linaro.org
[PMM: fixed style nits: tabs, long lines;
 pulled target_signal.h in from a later patch; it fits better here]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>