winch: Threads support

[saulecabrera]

The Wasm Threads proposal is considered Tier 1 according to Wasmtime's Tiers of support.

Winch currently doesn't support this proposal.

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[MarinPostma]

If my understanding is correct, we need to:

• we need to add to this macros the list of new instruction from thread proposal
• implement the new instructions for the aarch64 and x86_64 backend (in both masm.rs and asm.rs)
• probably add a compile time flag, like winch-thread that removes this flag while the feature is being developped

Is there anything else I'm missing?