Our pthreads shims don't check for reentrancy

[RalfJung]

libstd sets some flags on its pthreads locks that make pthreads check for reentrancy. Our shims for lock acquire/release don't do that though -- so code that should deadlock actually continues to run in Miri.

The following code demonstrates the issue:

use std::sync::Mutex;

fn main() {
    let m = Mutex::new(0);
    let _g1 = m.lock();
    let _g2 = m.lock();
    unreachable!();
}

[RalfJung]

But once parking_lot lands this might not matter any more -- then libstd will stop using pthread locks and we can remove the incorrect shims.

We'll probably have to stub out some other crazy stuff though.^^