Sam is working with me to try and get a basic x64 build running. I will post instructions when & if I get it to run. Your first step is to probably to build the filesystem. Osmc for x64 has not got a binary in the repo yet so that has to be modified & compiled manually. Additionally no partition wizards or guides exist so you need to understand how grub works to be able to boot the system.
Sam is doing all of the work but is obviously busy on mainstream production so I am just fiddling with code to try and get a working compile.
Will update with progress.
*I am not affiliated with OSMC or OSMC staff... I just love compiling.