aarch64-linux-user: Split out helpers for guest signal handling

aarch64-linux-user: Split out helpers for guest signal handling

Split out helpers from target_setup_frame and target_restore_sigframe
for dealing with general registers, fpsimd registers, and the end record.

When we add support for sve registers, the relative positions of
these will change.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20180303143823.27055-3-richard.henderson@linaro.org
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>