[AArch64] Add some CPU targets for "generic", A-53 and A-57.

Enables the clang driver to begin targeting specific CPUs. Introduced a
"generic" CPU which will ensure that the optional FP feature is enabled
by default when it gets to LLVM, without needing any extra arguments.
Cortex-A53 and A-57 are also introduced with tests, although backend
handling of them does not yet exist.