Fix undefined behavior when re-locking a mutex from the same thread

The only applies to pthread mutexes. We solve this by creating the
mutex with the PTHREAD_MUTEX_NORMAL type, which guarantees that
re-locking from the same thread will deadlock.

src/libstd/sys/windows/mutex.rs

@@ -64,6 +64,8 @@ impl Mutex {
             held: UnsafeCell::new(false),
         }
     }
+    #[inline]
+    pub unsafe fn init(&mut self) {}
     pub unsafe fn lock(&self) {
         match kind() {
             Kind::SRWLock => c::AcquireSRWLockExclusive(raw(self)),