Tighten the bounds on atomic Ordering in std::sys::unix::weak::Weak
This moves reading this from multiple SeqCst reads to Relaxed read + Acquire fence if we are actually going to use the data.
Would love to avoid the Acquire fence, but doing so would need Ordering::Consume, which neither Rust, nor LLVM supports (a shame, since this fence is hardly free on ARM, which is what I was hoping to improve).
r? ``@Amanieu`` (Sorry for always picking you, but I know a lot of people wouldn't feel comfortable reviewing atomic ordering changes)
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