Correct handling of enum attributes with g++

From OpenBSD's commit log:

This was responsible for memory corruption with recent versions
of Mesa where c and c++ code share a header with a packed enum type.

Reference:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=39219

Obtained from:	OpenBSD (CVS rev. 1.2)
MFC after:	1 week

contrib/gcc/cp/parser.c

@@ -10906,6 +10906,7 @@ cp_parser_enum_specifier (cp_parser* parser)
   if (cp_parser_allow_gnu_extensions_p (parser))
     {
       tree trailing_attr = cp_parser_attributes_opt (parser);
+      trailing_attr = chainon (trailing_attr, attributes);
       cplus_decl_attributes (&type,
 			     trailing_attr,
 			     (int) ATTR_FLAG_TYPE_IN_PLACE);