aarch64: Make more use of aarch64_feature_flags

aarch64: Make more use of aarch64_feature_flags

A previous patch added a aarch64_feature_flags typedef, to abstract
the representation of the feature flags.  This patch makes existing
code use the typedef too.  Hope I've caught them all!

gcc/
	* common/config/aarch64/aarch64-common.cc: Use aarch64_feature_flags
	for feature flags throughout.
	* config/aarch64/aarch64-protos.h: Likewise.
	* config/aarch64/aarch64-sve-builtins.h: Likewise.
	* config/aarch64/aarch64-sve-builtins.cc: Likewise.
	* config/aarch64/aarch64.cc: Likewise.
	* config/aarch64/aarch64.opt: Likewise.
	* config/aarch64/driver-aarch64.cc: Likewise.