We will need helpers that only make sense with AArch64. Add
helper-a64.{c,h} files as stubs that we can fill with these
helpers in the following patches.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>