fuzz: allow generating shared memories

[abrown]

wasm-smith v0.11 has support for generating shared memories when the
threads_enabled configuration flag is set. This change turns on that
flag occasionally.

[abrown]

@alexcrichton, @fitzgen: I am not really sure if this is the only place where the threads flag should be enabled to start generating shared memories--is there somewhere else that this makes sense to do?

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[alexcrichton]

I think you'll also need to add cfg.wasm_threads(...) to the actual Config generation. Additionally this feature may also need disabling in set_differential_config (but could be reenabled with v8 perhaps since we don't actually use threads). Otherwise though I think this is the main location.

Pending #4260 it might be good to run this for a bit locally to see if anything quickly pops up and/or introduce a purposeful bug/panic to make sure the fuzzer finds it at some point.

[abrown]

FYI: this was removed as a cleanup because line 350 below has config.simd_enabled = true;

[abrown]

This change is pending bytecodealliance/wasm-tools#639 and a new version of wasm-smith on crates.io.