Subzero: Make thread_local work under MacOS 10.6.

src/IceGlobalContext.h

Index: src/IceGlobalContext.h
diff --git a/src/IceGlobalContext.h b/src/IceGlobalContext.h
index 751147ee48502811e0f37eaf21c92d0dae0ad17c..e21c1ea30b13bd8f740774af1a23c86d875a9557 100644
--- a/src/IceGlobalContext.h
+++ b/src/IceGlobalContext.h
@@ -274,7 +274,7 @@ private:
   std::vector<ThreadContext *> AllThreadContexts;
   // Each thread has its own TLS pointer which is also held in
   // AllThreadContexts.
-  thread_local static ThreadContext *TLS;
+  ATTRIBUTE_TLS static ThreadContext *TLS;
 
   // Private helpers for mangleName()
   typedef llvm::SmallVector<char, 32> ManglerVector;