[ci] Update to GCC12

[salkinium]

Updating modm to GCC12 to keep up with the times.

• Merge osx-cross/avr-gcc@12 on macOS PR
• Merge docker-modm-build PR
• Update Install instructions
• Update Build System Documentation
• avrlibstdc++ needs to have its floating point math forwarding functions removed to prevent loops
• Fix additional compile flags for GCC12 are unknown to earlier compilers
  • Tested backward compatibility with GCC10
  • Tested new flags on GCC12
• Tested in Hardware
• Fix Python >3.8 multiprocessing issue with examples_compile.py and test all script
• Fix Python >3.8 multiprocessing and filesystem issue with docs generator
  • hangs the system, unclear why, so it's not fixed, instead using the previous container image where it still works

[chris-durand]

All CI issues should be fixed now, including the broken Mac OS build (if they merge the PR and avr-gcc does not fail because of --no-warn-rwx-segment).

[salkinium]

Thank you so much for fixing this, I fell asleep frustrated.

[salkinium]

Ok, avr-gcc 12 has been merged on macOS and I managed to make the new compile flags backward compatible with GCC10 and GCC11. Ran on as many examples in hardware as I was motivated, didn't find anything wrong. Should be good to go!

[salkinium]

Hm, the docs.modm.io generator script also uses multiprocessing and the Python 3.9 bug causes the job to hang. Have they not tested this?

[chris-durand]

Nice, thanks.

[salkinium]

I think there is an issue with the Ubuntu 22.04 update, rather than just Python multiprocessing code. I think specifically in the deduplication effort of the documentation generator, so I suspect something changed with the filesystem between 20.04 and 22.04.

[salkinium]

I've changed the container back to the previous one only for the build-docs-test. It doesn't use GCC anyways, only Python3 and I'm not motivated to debug the multiprocessing issues further.