[MBEDTLS] Switch license headers to the dual-licensed version. CORE-17155

This is the default going forward, and separating it out into its own
commit removes the noise from the following sync commit.

dll/3rdparty/mbedtls/threading.c

@@ -2,7 +2,30 @@
  *  Threading abstraction layer
  *
  *  Copyright (C) 2006-2015, ARM Limited, All Rights Reserved
- *  SPDX-License-Identifier: GPL-2.0
+ *  SPDX-License-Identifier: Apache-2.0 OR GPL-2.0-or-later
+ *
+ *  This file is provided under the Apache License 2.0, or the
+ *  GNU General Public License v2.0 or later.
+ *
+ *  **********
+ *  Apache License 2.0:
+ *
+ *  Licensed under the Apache License, Version 2.0 (the "License"); you may
+ *  not use this file except in compliance with the License.
+ *  You may obtain a copy of the License at
+ *
+ *  http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *  Unless required by applicable law or agreed to in writing, software
+ *  distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ *  WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *  See the License for the specific language governing permissions and
+ *  limitations under the License.
+ *
+ *  **********
+ *
+ *  **********
+ *  GNU General Public License v2.0 or later:
  *
  *  This program is free software; you can redistribute it and/or modify
  *  it under the terms of the GNU General Public License as published by
@@ -18,6 +41,8 @@
  *  with this program; if not, write to the Free Software Foundation, Inc.,
  *  51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
  *
+ *  **********
+ *
  *  This file is part of mbed TLS (https://tls.mbed.org)
  */