Add Teclast X80 Pro (EFI/x86_64 tablet)

This appears to be the first x86_64 device added. It boots with standard EFI instead of a custom bootloader, so installation is a little different (but documented on the wiki).

Because it's an Intel device with Intel graphics, this device is already effectively mainlined, with full 3D acceleration support as well. Unfortunately, our existing linux-postmarketos-* packages don't include enough things in the kernel config to boot on any real x86_64 machine I've tried. I spent quite a bit of time experimenting, trying to find the right options to make things work with no luck. So, I created linux-teclast-x80pro as a temporary workaround. It's an exact clone of linux-postmarketos-stable, but using the Arch Linux kernel config. This results in a very large kernel package, with lots of modules we don't need, but it does boot. At some point in the future the two packages should be merged, removing the clearly unnecessary modules.

Interestingly, very little of this work is specific to the Teclast device. I also tested this procedure on a similar cheap Windows tablet, and a Surface Pro 3 with success. I think that this work, with small modifications (probably just deviceinfo) is likely to work on most other EFI-booting x86_64 devices.