[Mips] Fix MIPS ABI name in the test. The name "64" is accepted by both gcc
and Clang drivers but internally LLVM/Clang use the name "n64".

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@211999 91177308-0d34-0410-b5e6-96231b3b80d8