I'd go a bit further. If you want full POSIX support, perhaps WASIX is the best alternative.
It's WASI preview 1 + many missing features, such as: threads, fork, exec, dlopen, dlsym, longjmp, setjmp, ...
I don't think that's accurate, although it's true that needs extra work to work properly in JS based environments.
You can already create threads in Wasm environments (we got even fork working in WASIX!). However, there is an upcoming Wasm proposal that adds threads support natively to the spec: https://github.com/WebAssembly/shared-everything-threads
Are you aware of WASI? WASI preview 1 provides a portable POSIXy interfance, while WASI preview 2 is a more complex platform abstraction beast.
(Keeping the platform separate from the assembly is normal and good - but having a common denominator platform like POSIX is also useful).