[X86] Add -mclwb/-mno-clwb command line arguments and __CLWB__ define to match gcc.

In the future, we should also add a clwb intrinsic to the backend, a frontend builtin, and an instrinsic header file.

llvm-svn: 294416
diff --git a/clang/lib/Basic/Targets.cpp b/clang/lib/Basic/Targets.cpp
index e9f5416..45ce531 100644
--- a/clang/lib/Basic/Targets.cpp
+++ b/clang/lib/Basic/Targets.cpp
@@ -3879,6 +3879,8 @@
     Builder.defineMacro("__GCC_HAVE_SYNC_COMPARE_AND_SWAP_16");
   if (HasCLFLUSHOPT)
     Builder.defineMacro("__CLFLUSHOPT__");
+  if (HasCLWB)
+    Builder.defineMacro("__CLWB__");
 
   // Each case falls through to the previous one here.
   switch (SSELevel) {