Handle std::sync::atomic::spin_loop_hint()
This PR adds support for `std::sync::atomic::spin_loop_hint()` by implementing the `llvm.x86.sse2.pause` intrinsic when the target is x86-based. It appears this is the first LLVM intrinsic in foreign_items, so I added a couple match blocks to handle it or fall through to the different OS-specific methods. I added a basic smoke test to `tests/run-pass/sync.rs`. I came across this by way of `crossbeam::utils::Backoff::spin()`, FWIW.
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