fix read-after-free type error in pthread_detach

fix read-after-free type error in pthread_detach

calling __unlock on t->exitlock is not valid because __unlock reads
the waiters count after making the atomic store that could allow
pthread_exit to continue and unmap the thread's stack and the object t
points to. for now, inline the __unlock logic with an unconditional
futex wake operation so that the waiters count is not needed.

once __lock/__unlock have been made safe for self-synchronized
destruction, we could switch back to using them.