linux-user/signal.c: Fix AArch64 big-endian FP register restore

Fix the loop restoring the FP registers from the signal frame to match
the one used when setting up the signal frame, so that it handles
TARGET_WORDS_BIGENDIAN being set.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>
