Add support for wasm32 platform

[rth]

It looks likely that WebAssembly will become an officially supported target for CPython https://bugs.python.org/issue40280 It's already mostly working on main.

Also, in Pyodide it's possible to create binary wheels for wasm32-unknown-emscripten (pyodide/pyodide#655) however when I try to upload such will on test pypi, the platform is rejected,

HTTPError: 400 Bad Request from https://test.pypi.org/legacy/
Binary wheel 'pyodide_test_wheel-2.2.0-cp39-pyodide18-emscripten_wasm32.whl' has an unsupported platform tag 'emscripten_wasm32'.

because it's not in the list of supported platforms here.

It seems likely that would be needed is support for at least,

emscripten_wasm32
wasi_wasm32

platforms (https://bugs.python.org/msg406985)

What would be the process to add those to allowed platforms? Thanks!

[di]

@tiran Can I get your thoughts here? I'm inclined to say we need a PEP to describe what these platforms actually mean, similar to the manylinux specs, but I admittedly don't know enough about the wasm ecosystem to say whether this is actually necessary.

[brettcannon]

We probably would need a PEP to clearly define what emscripten means there, whether compiler changes in emscripten/LLVM would ever cause wheels to stop functioning, etc.