This moves ARM and AArch64 target parsing into their
own files to enable future changes. They are still accessible
through TargetParser.h as before.
Several functions in AArch64 which were just forwarders to ARM
have been removed. All except AArch64::getFPUName were unused,
and that was only used in a test. Which itself was overlapping
one in ARM, so it has also been removed.