Convert low level thread creation to use wasi-threads API (thread_spawn)

The wasi-thread proposal is fairly far along now and, in particular, the thread-creation API has been worked through pretty well.

See https://github.com/WebAssembly/wasi-threads

In emscripten we have `__pthread_create_js` which takes the role of `thread_spawn` but we should be able convert to the standard API.

Doing this should allow `-sSTANDALONE_WASM` module to run in other runtimes such as wamr and wasmtime.