xen/mcelog: add PPIN to record when available

This is to augment commit 3f5a7896

 ("x86/mce: Include the PPIN in MCE
records when available").

I'm also adding "synd" and "ipid" fields to struct xen_mce, in an
attempt to keep field offsets in sync with struct mce. These two fields
won't get populated for now, though.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Juergen Gross <jgross@suse.com>