- Eliminate mutexattr_private and just set a bit in ptm_owner if the mutex
is recursive. This forces the slow path to be taken for recursive mutexes.
Overload an unused field in pthread_mutex_t to record whether or not it's
an errorcheck mutex.
- Streamline pthread_mutex_lock / pthread_mutex_unlock a bit more. As a
side effect makes it possible to have assembly stubs for them.