Optimize Once::doit when initialization is already completed

Load is much cheaper than fetch_add, at least on x86_64.

Verified with this test:

```
static mut o: one::Once = one::ONCE_INIT;
loop {
    unsafe {
        let start = time::precise_time_ns();
        let iters = 50000000u64;
        for _ in range(0, iters) {
            o.doit(||{
                println!("once!");
            });
        }
        let end = time::precise_time_ns();
        let ps_per_iter = 1000 * (end - start) / iters;
        println!("{} ps per iter", ps_per_iter);
    }
}
```

Test executed on Mac, Intel Core i7 2GHz. Result is 700ps per iteration
with patch applied, and 17000ps per iteration without patch.