Add experimental support for importing extension modules compiled for WASI

[brettcannon]

❯ ./run_wasi.sh -c "import importlib.machinery; print(importlib.machinery.EXTENSION_SUFFIXES)"
[]

See https://bytecodealliance.zulipchat.com/#narrow/stream/223391-wasm/topic/Python.20guest.20runtime.20and.20bindings/near/365948067 where this is being brought up.

[brettcannon]

This is probably blocked by upstream having the right support, else I don't see how trying to compile against dlopen() will succeed until it's available.

[dicej]

I have a prototype of dlopen support which I plan to add to wasi-libc once I've put it through some real-world tests (including Python extensions).

[brettcannon]

Since this depends on upstream providing functionality to make this work, I'm going to close this.