[date] Add ICU backend for timezone info behind a flag

test/mjsunit/icu-date-to-string.js

+// Copyright 2017 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: --icu-timezone-data
+// Environment Variables: TZ=Europe/Madrid LC_ALL=de
+
+assertEquals(
+   "Sun Dec 31 1989 00:00:00 GMT+0100 (Mitteleuropäische Normalzeit)",
+   new Date(1990, 0, 0).toString());
+
+assertEquals(
+   "Sat Jun 30 1990 00:00:00 GMT+0200 (Mitteleuropäische Sommerzeit)",
+   new Date(1990, 6, 0).toString());