std: add wasm64 to sync::Once and thread_parking atomics cfg guards

[Trivo25]

When targeting wasm64-unknown-unknown with atomics enabled, std::sync::Once and thread_parking fall through to the no_threads/unsupported implementations because the cfg guards only check for wasm32. This causes worker threads to panic with unreachable at runtime. The underlying futex implementations already handle both wasm32 and wasm64 correctly, only the cfg guards were missing wasm64.

I tested this manually with a multithreaded wasm64 application (o1js) compiled with -Z build-std=panic_abort,std and -C target-feature=+atomics,+bulk-memory,+mutable-globals

Related: #83879 #77839

Happy to adjust anything based on feedback

[rustbot]

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[joboet]

Thanks! Please apply Taiki's suggestions, then I'll approve merging.
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[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[Trivo25]

@rustbot ready

[tgross35]

(squash before merge please)