[Driver][Mips] MIPS ABI names "32" and "64" used as arguments of the "-mabi"

command line option only. Internally we convert them to the "o32" and "n64"
respectively. So we do not need to refer them anywhere after that conversion.

No functional changes.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@212096 91177308-0d34-0410-b5e6-96231b3b80d8