Omit the JS sync context if threads are not enabled

[kg]

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[ghost]

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Issue Details

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Author:	kg
Assignees:	-
Labels:	arch-wasm
Milestone:	-

[lambdageek]

I don't think FEATURE_WASM_THREADS is defined for non-CoreLib libraries by default - it's something we added by hand to the .csproj

also in .ts we can add some constant folding to encourage rollup to drop all the conditionals

[lambdageek]

/azp backport to release/7.0-rc1

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[maraf]

/backport to release/7.0-rc1

[github-actions]

Started backporting to release/7.0-rc1: https://github.com/dotnet/runtime/actions/runs/2889118587

[kg]

Verified that the threads sample works correctly now with @lambdageek's msbuild fix (thanks!)