[mypyc] Add support for building mypyc code on WASM

[emmatyping]

Description

This PR tweaks two things about how mypyc generates and builds C code to better support WebAssembly. First, we search sysconfig for the size of size_t, which works much better for cross-compiling. Second, newer versions of clang have -Wno-unused-but-set-variable and so it is added to the default list of arguments (this should probably land regardless the decision on merging this PR).

Test Plan

This PR depends on #13445. To test this PR, you can do the following:

assuming mypy checkout with both PRs applied, must be on Python 3.10(!)

$ pip install pyodide-build $ pyodide build --exports pyinit backend-args --global-option=--use-mypyc 

Note: you will get a warning about using --global-option, you can ignore it for now. I'm trying to find out why --build-option isn't working...

[emmatyping]

Blocked on python/typeshed#8572

[emmatyping]

Hmm I'm unsure why things disagree. It seems like sys.maxsize != sysconfig.get_config_var("SIZEOF_SIZE_T") on 32 bit Python on 64bit windows??

[emmatyping]

Ah yeah I messed up my math so it was setting SIZEOF_SIZE_T to 3 not 4 😅

This should be ready review now!

[emmatyping]

The latest commit 4ec39a8 enables outputting VT100 color codes for use in the web via things like https://www.npmjs.com/package/ansi-to-html. I guess I could directly output HTML but I figure that this is a bit more portable in case someone wants to run mypy under node or do something else. This is only enabled if you pass MYPY_FORCE_COLOR anyway.

[hauntsaninja]

Looks good, with the big caveat that I'm not sure I understand the exact circumstances when sysconfig's sizeof_size_t differs from sys.maxsize

[emmatyping]

@hauntsaninja

I'm not sure I understand the exact circumstances when sysconfig's sizeof_size_t differs from sys.maxsize

SIZE_OF_SIZE_T is only defined in UNIX environments, so we still rely on sys.maxsize on Windows. They only differ when you are cross compiling (like we would be for WASM)

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[emmatyping]

Oh this also needs #13445 to actually work (pyodide build uses the PEP 517 interface)