x86/apic: force phys mode if interrupt remapping is disabled

x86/apic: force phys mode if interrupt remapping is disabled

Cluster mode can only be used with interrupt remapping support, since
the top 16bits of the APIC ID are filled with the cluster ID, and
hence on systems where the physical ID is still smaller than 255 the
cluster ID is not. Force x2APIC to use physical mode if there's no
interrupt remapping support.

Note that this requires a further patch in order to enable x2APIC
without interrupt remapping support.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>