Use aarch64_features to describe register features in target descriptions.

Use aarch64_features to describe register features in target descriptions.

Replace the sve bool member of aarch64_features with a vq member that
holds the vector quotient.  It is zero if SVE is not present.

Add std::hash<> specialization and operator== so that aarch64_features
can be used as a key with std::unordered_map<>.

Change the various functions that create or lookup aarch64 target
descriptions to accept a const aarch64_features object rather than a
growing number of arguments.

Replace the multi-dimension tdesc_aarch64_list arrays used to cache
target descriptions with unordered_maps indexed by aarch64_feature.