[wasm][asm.js] Store foreign init function for asm.js as internal field.

test/mjsunit/asm/regress-681236.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: --validate-asm
+
+function __f_3() {
+  "use asm";
+  function __f_15(__v_7) {
+    __v_7 = __v_7|0;
+  }
+  return {__f_15: __f_15};
+}
+assertEquals(['__f_15'], Object.getOwnPropertyNames(__f_3()));