actually implement proc:::lwp-exit probe

MFC after:	4 days

sys/kern/kern_thread.c

@@ -473,6 +473,7 @@ thread_exit(void)
 	KASSERT(p != NULL, ("thread exiting without a process"));
 	CTR3(KTR_PROC, "thread_exit: thread %p (pid %ld, %s)", td,
 	    (long)p->p_pid, td->td_name);
+	SDT_PROBE0(proc, , , lwp__exit);
 	KASSERT(TAILQ_EMPTY(&td->td_sigqueue.sq_list), ("signal pending"));
 
 #ifdef AUDIT