pthreads: Use _exit to terminate the tst-stdio1 test

Previously, the exit function was used, but this causes the test to
block (until the timeout) once exit is changed to lock stdio streams
during flush.

sysdeps/pthread/tst-stdio1.c

@@ -46,7 +46,7 @@ do_test (void)
       _exit (1);
     }
 
-  delayed_exit (1);
+  delayed__exit (1);
   xpthread_join (th);
 
   puts ("join returned");