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.