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

[sbc100]

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.