cmake: match amd64 for x86 support
cmake: match amd64 for x86 support

This is used on kFreeBSD, and possibly by FreeBSD as well.

chr-engwer/Vc,chr-engwer/Vc,VcDevel/Vc,VcDevel/Vc,VcDevel/Vc,VcDevel/Vc,chr-engwer/Vc