Make Triple::parseARMArch use ARMTargetParser

Simplifying Triple::parseARMArch, leaving all the parsing to ARMTargetParser.

This commit also adds AArch64 detection to ARMTargetParser canonicalization,
and a two RedHat arch names (v{6,7}hl, meaning hard-float / little-endian).

Adding enough unit tests to cover the basics. Clang checks fine.

llvm-svn: 237902