x86: apic: Fix mismerge, add arch_probe_nr_irqs() again

Merge commit aef55d49 mis-merged io_apic.c so we lost the
arch_probe_nr_irqs() method.

This caused subtle boot breakages (udev confusion likely
due to missing drivers) with certain configs.

Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Yinghai Lu <yinghai@kernel.org>
LKML-Reference: <20100207210250.GB8256@jenkins.home.ifup.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>