x86 mce: fix c/s 17968 for 32-on-64

x86 mce: fix c/s 17968 for 32-on-64

32-on-64 aspects were not properly considered. Add respective
checking, and adjust structure layouts for the cases where the
checking pointed out issues.

Also,
- fix a potential memory corruption issue (do_mca() could write beyond
  log_cpus' end if the guest specified less than the number of online
  CPUs
- there is no reason to make the (not even properly prefixed)
  definitions in xen/public/arch-x86/xen-mca.h globally visible by
  including the file from xen/public/arch-x86/xen.h.

Signed-off-by: Jan Beulich <jbeulich@novell.com>