libmach_amd64: refresh darwin thread list each time we get asked.

R=r
DELTA=1  (1 added, 0 deleted, 0 changed)
OCL=15982
CL=15993

src/libmach_amd64/darwin.c

@@ -343,6 +343,7 @@ procthreadpids(int id, int **thread)
 	if(t == nil)
 		return -1;
 	pid = t->pid;
+	addpid(pid, 1);	// force refresh of thread list
 	n = 0;
 	for(i=0; i<nthr; i++)
 		if(thr[i].pid == pid)