Revert "Revert "std: Re-enable at_exit()""

This reverts commit aec67c2ee0.

src/test/run-fail/rt-set-exit-status-panic2.rs

@@ -35,7 +35,7 @@ fn r(x:int) -> r {
 
 fn main() {
     error!("whatever");
-    let _t = thread::spawn(move|| {
+    let _t = thread::scoped(move|| {
       let _i = r(5);
     });
     panic!();

src/test/run-pass/unique-send-2.rs

@@ -25,7 +25,7 @@ pub fn main() {
     let _t = (0..n).map(|i| {
         expected += i;
         let tx = tx.clone();
-        thread::spawn(move|| {
+        thread::scoped(move|| {
             child(&tx, i)
         })
     }).collect::<Vec<_>>();