GM: brake if rolling back

[sshane]

On certain steep hills, max gas won't be enough to move forward, so the PCM enters standstill and we start rolling back. Since vEgo is absolute, we don't know about this.

Some attempts:

• https://github.com/commaai/openpilot/pull/28245
• https://github.com/commaai/openpilot/pull/28214

after thinking about it more, I think the correct and smaller fix is to enter stopping state if the car enters standstill

• [ ] use signed vehicle speed