AArch64: add missing Apple CPU names and use them by default.

AArch64: add missing Apple CPU names and use them by default.

Apple's CPUs are called A7-A13 in official communication, occasionally with
weird suffixes which we probably don't need to care about. This adds each one
and describes its features. It also switches the default CPU to the canonical
name for Cyclone, but leaves legacy support in so that existing bitcode still
compiles.