xen/arch: Switch to new byteorder infrastructure

xen/arch: Switch to new byteorder infrastructure

This needs to be done in several steps, because of common vs arch issues.
Start by using the new common infrastructure inside the arch infrastructure.

libelf-private.h is awkward, and the only thing in Xen using swab??()
directly.  It needs updating at the same time.

Signed-off-by: Lin Liu <lin.liu@citrix.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>