ARM port reentrancy: Close per-thread stdout when runs to completion.

ports/arm/atomport.c

@@ -92,6 +92,7 @@ static void thread_shell (void)
     }
 
     /* Clean up after thread completion */
+    fclose (stdout);
     _reclaim_reent (&(curr_tcb->port_priv.reent));
 
     /* Thread has run to completion: remove it from the ready list */

ports/arm/syscalls.c

@@ -76,7 +76,7 @@ extern int _exit(int val) __attribute__((weak));
  */
 int _close(int file)
 {
-    return -1;
+    return 0;
 }