Rewrite varargs type descriptors to match gc compiler.

Change empty interfaces to have NULL methods_ field.

R=iant
https://golang.org/cl/204050
diff --git a/go/unsafe.cc b/go/unsafe.cc
index be5b7cd..6e78dcd 100644
--- a/go/unsafe.cc
+++ b/go/unsafe.cc
@@ -81,8 +81,7 @@
     this->add_named_object(no);
 
   // Typeof.
-  Typed_identifier_list* empty_methods = new Typed_identifier_list();
-  Type* empty_interface = Type::make_interface_type(empty_methods, bloc);
+  Type* empty_interface = Type::make_interface_type(NULL, bloc);
   Typed_identifier_list* parameters = new Typed_identifier_list;
   parameters->push_back(Typed_identifier("i", empty_interface, bloc));
   results = new Typed_identifier_list;