Add support for c++20 modules using GCC

[TroyKomodo]

Clang was supported via #2516

[Felixoid]

There's a failure in cmake-modules test, btw https://github.com/mozilla/sccache/actions/runs/23459356336/job/68257326892?pr=2581#step:10:38

The @ symbol in the cmake 4.X causes non-cacheable result. In #2649 it's suddenly fixed by using 3.x version, but it's something that needs to be addressed too.