xen: Introduce a header to store common linker scripts content

xen: Introduce a header to store common linker scripts content

Both x86 and arm linker scripts share quite a lot of common content.
It is difficult to keep syncing them up, thus introduce a new header
in include/xen called xen.lds.h to store the internals mutual to all
the linker scripts.

Include this header in linker scripts for x86 and arm.
This patch serves as an intermediate step before populating xen.lds.h
and making use of its content in the linker scripts later on.

Signed-off-by: Michal Orzel <michal.orzel@arm.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Julien Grall <jgrall@amazon.com>