x86, apic: streamline the ->multi_timer_check() quirk

only NUMAQ uses this quirk: to prevent the timer IRQ from being added
on secondary nodes.

All other genapic templates can have a NULL ->multi_timer_check()
callback.

Also, extend the generic code to treat a NULL pointer accordingly.

Signed-off-by: Ingo Molnar <mingo@elte.hu>