xen/x86: Correct mandatory and SMP barrier definitions

xen/x86: Correct mandatory and SMP barrier definitions

Barriers are a complicated topic, a source of confusion, and their incorrect
use is a common cause of bugs.  It really doesn't help when Xen's API is the
same as Linux, but its ABI different.

Bring the two back in line, so programmers stand a chance of actually getting
their usage correct.

Drop the links in the comment, both of which are now stale.  Instead, refer to
the vendor system manuals in a generic way.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Dario Faggioli <dario.faggioli@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>