The producers of ABC's new sitcom Cavemen, based on a series of popular advertisements for the insurance company Geico, were ready to defend themselves against charges of rampant commercialism. But the charge that their show about put-upon Cro-Magnons in the modern world has racist undertones took them by surprise. ''I actually didn't know we would catch so much hell,'' Cavemen writer Joe Lawson ruefully told a room full of highly critical critics at a gathering of North American television writers here Wednesday. ``That's a pleasant surprise.'' Cavemen, which features shaggy Cro-Magnons trying to make their way through a hostile Homo...