pulley: Disable the WebAssembly threads proposal

[alexcrichton]

Forcibly disable the threads proposal for Pulley. This is done because I don't believe there's any way that we can, in Rust, implement non-atomic loads and stores in a manner that isn't undefined behavior. Until this is available leave this proposal as flagged as unsupported.

[alexcrichton]

I've also opened a topic on the rust-lang/rust zulip for this

[github-actions]

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