Add support for __GCC_HAVE_SYNC_COMPARE_AND_SWAP_{1,2,4,8} on x86.

This fixes std::thread with libstdc++.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@178816 91177308-0d34-0410-b5e6-96231b3b80d8