[regexp] implement RegExp.prototype.toString for non-RegExp receiver.

test/mjsunit/es6/regexp-tostring.js

+// Copyright 2016 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.
+
+var log = [];
+
+var fake = {
+  pattern: function() {
+    log.push("p");
+    return {
+      toString: function() {
+        log.push("ps");
+        return "pattern";
+      }
+    };
+  },
+  flags: function() {
+    log.push("f");
+    return {
+      toString: function() {
+        log.push("fs");
+        return "flags";
+      }
+    };
+  }
+}
+
+assertThrows(() => RegExp.prototype.toString.call(1));
+assertEquals("/pattern/flags", RegExp.prototype.toString.call(fake));
+assertEquals(["p", "ps", "f", "fs"], log);