std.Io.Threaded: implement Mutex and Condition

[andrewrk]

Extracted from #25592.

Unfortunately NetBSD does not have futexes, so Mutex and Condition must be based on pthread_mutex and pthread_condition. This will mean reserving 48 bytes into Io.Mutex for storing state.

    if (native_os == .netbsd) @panic("TODO");

Alternately (preferably), they could be based on _lwp_park and _lwp_unpark, which is how the pthreads API is implemented internally on NetBSD, but in this case roughly the same amount of storage for state will be needed in Io.Mutex.

[alexrp]

This also affects illumos: #7152 (comment)