Add I128 atomic support to the x64 backend

[beetrees]

Rust is currently in the process of stabilising 128-bit atomics (rust-lang/rust#99069 (comment)), but Cranelift (and therefore rustc_codegen_cranelift) doesn't currently support 128-bit atomics.

This PR adds IR support for 128-bit atomics and implements lowering them for the x64 backend using cmpxchg16b.

[github-actions]

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This issue or pull request has been labeled: "cranelift", "cranelift:area:x64", "cranelift:meta", "fuzzing", "isle", "wasmtime:api", "wasmtime:config"

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[fitzgen]

Very nice, thanks!

[beetrees]

I think CI might be stuck? Test macOS x86_64 (wasmtime-cli) seems to have been stuck on cargo fetch --locked for over an hour.

[beetrees]

Never mind, it's managed to move on to the next step.

[beetrees]

It seems to have got stuck in the ./ci/run-tests.sh step now (first it was stuck at the very start of the step for around 30 mins, and now it's stuck at Compiling bytesize v1.3.0).

[fitzgen]

I've been seeing a lot of network issues in github actions the last week or so