Add unittests to {ARM | AArch64}TargetParser.

Add unittest to {ARM | AArch64}TargetParser,and by the way correct problems as below:
1.Correct a incorrect indexing problem in AArch64TargetParser. The architecture enumeration
 is shared across ARM and AArch64 in original implementation.But In the code,I just used the
 index which was offset by the ARM, and this would index into the array incorrectly. To make
 AArch64 has its own arch enum,or we will do a lot of slowly iterating.
2.Correct a spelling error. The parameter of llvm::AArch64::getArchExtName.
3.Correct a writing mistake, in llvm::ARM::parseArchISA.

Differential Revision: https://reviews.llvm.org/D21785

llvm-svn: 276957