Add remaining @@toStringTag symbols to builtins

test/mjsunit/es6/math.js

+// Copyright 2014 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --harmony-tostring
+
+function testMathToString() {
+  assertEquals('[object Math]', "" + Math);
+  assertEquals("Math", Math[Symbol.toStringTag]);
+  var desc = Object.getOwnPropertyDescriptor(Math, Symbol.toStringTag);
+  assertTrue(desc.configurable);
+  assertFalse(desc.writable);
+  assertEquals("Math", desc.value);
+}
+testMathToString();