Simplify ARMTargetParser::getArchSynonym

Summary:
1) The only caller, ARMTargetParser::parseArch, uses the results for an "endswith" test; so, including the "arm" prefix into the result is unnecessary.
2) Most ARMTargetParser::parseArch callers pass it the output from ARMTargetParser::getCanonicalArchName; so, make this behaviour the default. Then, including the "arm" prefix into the cases is unnecessary.

Reviewers: rengolin

Reviewed By: rengolin

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D10249

llvm-svn: 239099