Fix for bug 23429.
Fix for bug 23429.

The macros for gcc atomic compare and swaps are defined for Power8 CPU's since
the functionality is provided in the back end.


git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@236848 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang