std::os::rand::imp::is_getrandom_available atomics use is racy

[talchas]

is_getrandom_available relies on the ordering of reads and writes that use Ordering::Relaxed to prevent races, which is incorrect. The failure mode would be reading the initial false value of GETRANDOM_AVAILABLE, and thus not using getrandom(2) when it is actually available. Either GETRANDOM_CHECKED should use an acquire load and a release store, or it could just use Once.

[klutzy]

Ah! The problem is that two atomic writes can be swapped, right? I'll rewrite it with one variable with three states (notchecked / available / unavailable) anyway.

[talchas]

Yeah, that is the problem, and that would also fix it.