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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>