Introduce the main header for conventional variations of Xen. Some
setups are unavailable on some hardware architectures. Some hardware
architectures require differing memory allocation types.
The original author of FreeBSD on Xen/ARM code brought the whole header
in as a large blob. Introducing this header earlier allows showing how
ARM varies from what x86 has.
The content was created in 2014, even though the Git expression of it
was created in 2021.