xen: sync some headers with xen tree

To be able to use an initially unmapped initrd with xen the following
header files must be synced to a newer version from the xen tree:

include/xen/interface/elfnote.h
include/xen/interface/xen.h

As the KEXEC and DUMPCORE related ELFNOTES are not relevant for the
kernel they are omitted from elfnote.h.

Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>