Implement optimized dispatch_once

[SSheldon]

dispatch_once_f is redefined as:

void _dispatch_once_f(dispatch_once_t *predicate, void *context, dispatch_function_t function)
{
    if (__builtin_expect(*predicate, ~0l) != ~0l) {
        dispatch_once_f(predicate, context, function);
    }
}

This requires the expect builtin, which isn't exposed as an intrinsic in Rust. Implementing this would be possible if rust-lang/rfcs#918 is accepted and implemented.

[SSheldon]

This is probably doable with the #[cold] attribute: https://doc.rust-lang.org/stable/reference.html#function-only-attributes