[GOOGLE] Avoid emitting duplicated weakref

[Dehao Chen]

This patch fixes a bug when two weakref symbols are mapped to a same
assembler name.

Testing on going.

OK for google branches if test is fine?

Thanks,
Dehao

Index: gcc/varasm.c
===================================================================
--- gcc/varasm.c (revision 199844)
+++ gcc/varasm.c (working copy)
@@ -5502,6 +5502,10 @@ do_assemble_alias (tree decl, tree target)
   if (TREE_ASM_WRITTEN (decl))
     return;

+  if (lookup_attribute ("weakref", DECL_ATTRIBUTES (decl))
+      && TREE_ASM_WRITTEN (DECL_ASSEMBLER_NAME (decl)))
+    return;
+
   /* We must force creation of DECL_RTL for debug info generation, even though
      we don't use it here.  */
   make_decl_rtl (decl);