break on attempt to build amd64 on i386

This isn't possible. (At least not for VM images.) Normally, I know this. But I lost some time to figure out what I just attempted myself.

If it doesn't break on attempt to build amd64 on i386 at that point, you'll later get an `mount unknown executable format` (or something like that) error, which isn't trivial to make head or tail of.

Please check if this commit is sane.