[Mips] Add two new aliases for MIPS ABI names 32 (means o32 abi) and 64
(means n64 abi) to improve compatibility with GNU tools.
Patch by Jia Liu <proljc@gmail.com>.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@176187 91177308-0d34-0410-b5e6-96231b3b80d8