compile: add type conversion in Type::gc_symbol_pointer

Wrap the return from Type::gc_symbol_pointer with a type conversion to
    uintptr, since the values returned are stored into structure fields
    with that type.
    
    Reviewed-on: https://go-review.googlesource.com/34625

From-SVN: r243978

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-d9be5f5d7907cbc169424fe2b8532cc3919cad5b
+ebe9d824adca053066837b8b19461048ced34aff
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

gcc/go/gofrontend/types.cc

@@ -2138,7 +2138,10 @@ Type::gc_symbol_pointer(Gogo* gogo)
   Location bloc = Linemap::predeclared_location();
   Bexpression* var_expr =
       gogo->backend()->var_expression(t->gc_symbol_var_, VE_rvalue, bloc);
-  return gogo->backend()->address_expression(var_expr, bloc);
+  Bexpression* addr_expr =
+      gogo->backend()->address_expression(var_expr, bloc);
+  Btype* ubtype = Type::lookup_integer_type("uintptr")->get_backend(gogo);
+  return gogo->backend()->convert_expression(ubtype, addr_expr, bloc);
 }
 
 // A mapping from unnamed types to GC symbol variables.