Have Tails Installer use syslinux-amd64 on amd64 platforms

On amd64 platforms we’ll need to explictly tell the installer to use the syslinux-amd64 binary.

Feature Branch: 451f:liveusb-creator/feature/jessie_syslinux64_2

Original created by @u on 9609 (Redmine)

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information