xen: introduce Kconfig function alignment option

xen: introduce Kconfig function alignment option

And use it to replace CODE_ALIGN in assembly.  This allows to generalize the
way the code alignment gets set across all architectures.

No functional change intended.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Michal Orzel <michal.orzel@amd.com>
Acked-by: Shawn Anastasio <sanastasio@raptorengineering.com>