add multilib flag for nvidia-cg-toolkit on amd64

Hi,

Small changes which adds the multilib flag for nvidia-cg-toolkit on amd64.

Further more i suggest removing following:
```
		steamgames_source_engine? (
				video_cards_fglrx? ( >=x11-drivers/ati-drivers-12.8 )
			)
```
There are no such old ati-drivers anymore in main-tree. The oldest one is 13.1, thus this dependency is rather useless.

Regards,
Michael