Support synchronization primitives on Windows

[RalfJung]

Thanks to @drmeepster we now do support basic concurrency and atomics on Windows (#2231). However, we do not support all of the standard library concurrency primitives. Based on my testing:

• Mutex
• RwLock
• Condvar
• thread parking

The fact that we don't support thread parking (which is needed by std::sync::mpsc) unfortunately blocks rust-lang/rust#103681.

[RalfJung]

@drmeepster if I recall, you actually had a first implementation of at least thread parking. Is that code still available somewhere?

[RalfJung]

Looks like the code still exists here, in particular in 2cd61e6.

[RalfJung]

Thread parking is implemented now, Condvars are still missing.

[beepster4096]

Opened #2638 for condvars