fix regression in pthread_exit

commit d26e0774a5 moved the detach state
transition at exit before the thread list lock was taken. this
inadvertently allowed pthread_join to race to take the thread list
lock first, and proceed with unmapping of the exiting thread's memory.

we could fix this by just revering the offending commit and instead
performing __vm_wait unconditionally before taking the thread list
lock, but that may be costly. instead, bring back the old DT_EXITING
vs DT_EXITED state distinction that was removed in commit
8f11e6127f, and don't transition to
DT_EXITED (a value of 0, which is what pthread_join waits for) until
after the lock has been taken.

src/internal/pthread_impl.h

@@ -68,7 +68,8 @@ struct pthread {
 };
 
 enum {
-	DT_EXITING = 0,
+	DT_EXITED = 0,
+	DT_EXITING,
 	DT_JOINABLE,
 	DT_DETACHED,
 };

src/thread/pthread_create.c

@@ -156,6 +156,7 @@ _Noreturn void __pthread_exit(void *result)
 	}
 
 	/* Wake any joiner. */
+	a_store(&self->detach_state, DT_EXITED);
 	__wake(&self->detach_state, 1, 1);
 
 	/* After the kernel thread exits, its tid may be reused. Clear it