make LIBCXXABI_NORETURN be the right attribute

llvm-svn: 136680
diff --git a/libcxxabi/include/cxxabi.h b/libcxxabi/include/cxxabi.h
index 79b5597..196e230 100644
--- a/libcxxabi/include/cxxabi.h
+++ b/libcxxabi/include/cxxabi.h
@@ -18,7 +18,7 @@
 #include <stddef.h>
 #include <stdint.h>
 
-#define LIBCXXABI_NORETURN
+#define LIBCXXABI_NORETURN  __attribute__((noreturn))
 
 namespace std {
     class type_info; // forward declaration