rustc_thread_pool: Make `CoreLatch::set` use `SeqCst` instead of `AcqRel`
Every other modification of this variable uses `SeqCst`, which is justified in the sleep README. This particular choice of `AcqRel` was not discussed during its addition in rayon-rs/rayon#746, nor rayon-rs/rfcs#5, so I suspect was simply an oversight from earlier development. We probably do want this to participate in the same sequential consistency.
The only other ordering difference is `CoreLatch::probe`'s load with `Acquire`, which should be fine because this doesn't need consistency with the sleep counters.
See also rayon-rs/rayon#1297. As I commented there, I think in practice this would be quite rare to cause any problems, but it *could* be a source of non-deterministic bugs on targets with weak memory ordering.
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