[mips] Fix ELF header flags.

As opposed to GCC/GAS the default ABI for Mips64 is n64.
Compatibility bit should be set if o32 ABI is used when targeting Mips64.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200332 91177308-0d34-0410-b5e6-96231b3b80d8