GM: add checksum to button message

[JasonJShuler]

Description It appears some components reject button messages without the checksum. This PR adds the checksum to button spam messages

Verification As there are a limited number of possible values, it was possible to compare the output of this formula with all of them. Tested the checksum with different button values and all the rolling counters.

• [ ] Deal with bit 24. It only appears to be "1" in cars with ACC. The checksum formula needed to be changed based on a non-ascm car since we don't currently send this value. I would like to have a single formula that works in both scenarios