Add <mstd_mutex>

Add add-standard-as-possible version of C++11 <mutex>.

A lot of the stuff in there is generic, but the actual mutex classes and
call_once need to interface with the OS.

For those, they're not available in ARMC5 or IAR; retargetting would be
necessary for ARMC6 and GCC, and I've yet to investigate how whether
that's possible. So for now I'm using local implementations.

Although `Mutex` in principle could support `timed_mutex` and
`recursive_timed_mutex`, we don't have `chrono` for the time parameters,
so hold off for now.

For the generic stuff like mstd::unique_lock, they are aliased to
std::unique_lock where possible.

Author: kjbracey

platform/cxxsupport/TOOLCHAIN_ARMC5/mutex

@@ -0,0 +1,45 @@
+/* mbed Microcontroller Library
+ * Copyright (c) 2019 ARM Limited
+ * SPDX-License-Identifier: Apache-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.
+ */
+#ifndef __mutex
+#define __mutex
+
+// Just go straight to the main file - we also need IAR implementation, so do it there
+#include <mstd_mutex>
+
+// And then pull it all into our std
+namespace std {
+using mstd::defer_lock;
+using mstd::defer_lock_t;
+using mstd::try_to_lock;
+using mstd::try_to_lock_t;
+using mstd::adopt_lock;
+using mstd::adopt_lock_t;
+
+using mstd::lock_guard;
+using mstd::unique_lock;
+using mstd::scoped_lock;
+using mstd::try_lock;
+using mstd::lock;
+
+using mstd::once_flag;
+using mstd::call_once;
+
+using mstd::mutex;
+using mstd::recursive_mutex;
+}
+
+#endif /* __mutex */