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.