gh-132097: use a macro for semantically casting function pointers

[picnixz]

This is typically useful when one needs to find if a cast is needed or not.

• Issue: UBSan: runtime undefined behaviours when using -fsanitize=undefined -fno-sanitize-recover #132097

[picnixz]

!buildbot EMScripten

[bedevere-bot]

The regex 'EMScripten' did not match any buildbot builder. Is the requested builder in the list of stable builders?

[vstinner]

LGTM

[picnixz]

Hum, ideally I wanted to be sure that this change won't be bad for EMScripten but hopefully it won't.

@hoodmane Can you confirm that this won't be breaking EMScripten builds? (and do we have a build bot for it?)

[hoodmane]

It looks fine to me. No build bot still, we have to merge #132092 first.

Thanks for the ping @picnixz !