request an object mask when allocating a singleton

Singletons may have embedded object references, and if they are
allocated at fixed memory locations (e.g. if they are larger than
64KB), they must have object masks so the garbage collector knows were
to find said references.

src/type-generator.cpp

@@ -1911,7 +1911,7 @@ writeConstructors(Output* out, Object* declarations)
 
       out->write(")\n{\n");
 
-      bool hasObjectMask = false;
+      bool hasObjectMask = strcmp(typeName(o), "singleton") == 0;
       for (MemberIterator it(o); it.hasMore();) {
         Object* m = it.next();
         if (m->type == Object::Scalar