threads: add feature flags

[abrown]

This change adds the minimal, internal flags necessary for testing the shared-everything-threads proposal. It chooses to skip the user-facing CLI flags for now and, as suggested, keeps the Cargo threads feature as the sole feature flag gating compilation of threads-related code, which will eventually include shared-everything-threads.

[github-actions]

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[alexcrichton]

What do you think about using the preexisting threads Cargo feature for this? That's mostly avoiding runtime deps of threads/libstd/etc and having that gate both threads and shared-everything-threads seems reasonable to me

[alexcrichton]

Oh and do directly answer this:

The feedback I'm look for is: what we should be gating with this new Cargo feature?

I wrote up some basic thoughts here but the tl;dr; IMO is "anything that requires a runtime dependency" aka something outside of libcore. In that sense the current threads proposal is gated because it requires mutexes/etc to implement atomic notify/wait. I don't think that shared-everything-threads will require any more than threads, though, but any nontrivial dependency above the core crate should be gated.

(or do you have specific things you're curious about too?)

[abrown]

The latest commits remove the CLI flags since we probably don't want to expose that yet to users and adopt @alexcrichton's suggestion to use the threads Cargo feature instead of creating a new, specialized shared-everything-threads feature. What is now added is all the internal flag bits necessary for running WAST tests, the absence of which is one blocker to #10518. I'm going to alter the PR message to reflect this so we get a more accurate commit message in the Git history.

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[alexcrichton]

Technically this won't actually work if the test actually used shared-everything-threads in the sense that this block needs to be updated for fuzzing to propagate the need of shared-everything-threads to wasmtime::Config, and that's going through CLI flags which means this'll also have to add -Wshared-everything-threads.

That being said such a change is a superset of what you've got here, and this seems fine otherwise.