Move parseSubArch to ARMTargetParser. NFC

Using getCanonicalArchName() is the right way to parse ARM arch names.

Mapping ARMTargetParser IDs to Triple Arch IDs is temporary, until they
are merged into a TargetDescription class.

This was the last LLVM FIXME to move things to ARMTargetParser. Now on
to Clang and beyond.

llvm-svn: 238110