For WASM, that's what WASI is all about, yes?

And for Cosmopolitan Libc, there's documented Functions:

https://justine.lol/cosmopolitan/functions.html

And if you want to see things beyond a trivial hello world, you can check out some examples:

https://github.com/shmup/awesome-cosmopolitan

https://github.com/burggraf/awesome-cosmo

Or you can see a pretty big list of pre-compiled Actually Portable Executables here:

https://cosmo.zip/pub/cosmos/bin/