arch-armv8-4a.inc: add tune include for armv8.4a

Add tune include for armv8.4a. This adds support only for bare armv8.4a
(and for crypto extension). There is no support for additional
instructions added by architecture extensions (except the main crypto
extension support).

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>