Add `rustc_diagnostic_item` to `sys::Mutex` methods
For an ongoing project for adding a concurrency model checker to Miri we need to be able to intercept locking/unlocking operations on standard library mutexes.
This PR adds diagnostic items to the relevant calls `lock`, `try_lock` and `unlock` for the `sys::Mutex` implementation on the targets we care about.
This PR also makes the internals of `pthread::Mutex` less public, to reduce the chance of anyone locking/unlocking a mutex without going through the intercepted methods.
r? ``@RalfJung``
e7ef07a523