x86: mce: Don't touch THERMAL_APIC_VECTOR if no active APIC present

If APIC was disabled (for some reason) and as result
it's not even mapped we should not try to enable thermal
interrupts at all.

Reported-by: Simon Holm Thøgersen <odie@cs.aau.dk>
Tested-by: Simon Holm Thøgersen <odie@cs.aau.dk>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
LKML-Reference: <20090615182633.GA7606@lenovo>
Signed-off-by: Ingo Molnar <mingo@elte.hu>