target-arm: Fix aarch64 vec_reg_offset

Since CPUARMState.vfp.regs is not 16 byte aligned, the ^ 8 fixup used
for a big-endian host doesn't do what's intended.  Fix this by adding
in the vfp.regs offset after computing the inter-register offset.

Signed-off-by: Richard Henderson <rth@twiddle.net>
Message-id: 1481085020-2614-2-git-send-email-rth@twiddle.net
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>