std: use futex-based locks on Fuchsia
This switches `Condvar` and `RwLock` to the futex-based implementation currently used on Linux and some BSDs. Additionally, `Mutex` now has its own, priority-inheriting implementation based on the mutex in Fuchsia's `libsync`. It differs from the original in that it panics instead of aborting when reentrant locking is detected.
````@rustbot```` ping fuchsia
r? ````@m-ou-se````
c5df2f02d3