Use getArch instead of getArchName.

The darwin change should be a nop since Triple::getArchTypeForDarwinArchName
doesn't know about amd64.

If things like amd64-mingw32 are to be rejected, we should print a error
earlier on instead of silently using the wrong abi.

Remove old comment that looks out of place, this is "in clang".

llvm-svn: 165368