Support cpu dimension in the engine repo, so #101871 but in the engine.

Also remove mac_model from the engine .ci.yaml in cases where it was only being used to enforce device architecture.
https://github.com/flutter/engine/blob/47c8a6acf9f37fcb4e7dce4cddc5a1590b83b715/.ci.yaml#L56