Add missing pragma GCC system_header

Author: dalg24

libcxx/include/__atomic/floating_point_helper.h

@@ -13,6 +13,10 @@
 #include <__type_traits/is_floating_point.h>
 #include <__type_traits/is_same.h>
 
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 #if _LIBCPP_STD_VER >= 20