[mips] Add _GCC_HAVE_SYNC_COMPARE_AND_SWAP macros. This fixes the 'pure virtual function called' failure with ThreadPool in a clang-built clang. This fixes the llvm-mips-linux builder. git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@256240 91177308-0d34-0410-b5e6-96231b3b80d8