pub-headers: reduce C99 dependencies

pub-headers: reduce C99 dependencies

For consumers not using (fully) C99-aware compilers, limit the number
of places where tweaking of the headers would be necessary: Introduce
and use xen_mk_ullong(), allowing its helper macro to be overridden at
once.

For now don't touch public/io/, which also has a few offenders.

The need to include xen.h in hvm/e820.h demonstrates that it is a bad
idea to include public headers first thing - arch/x86/hvm/mtrr.c needs
adjustment just because of this.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Julien Grall <julien.grall@arm.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>