The end goal of the series is to change the definition of aarch64_feature_flags from a uint64_t typedef to a class with 128 bits of storage. This class is a new template bitmap type that uses operator overloading to mimic the existing integer interface as much as possible.
The changes are mostly in the backend, but patch 10/11 introduces this new bitmap type in the middle end. Committed as approved by Kyrill and Richard S, with minor changes to patches 04, 08 and 10 as requested by Richard.
