x86: make /proc/stat account for all interrupts

LAPIC interrupts, which don't go through the generic interrupt handling
code, aren't accounted for in /proc/stat. Hence this patch adds a
mechanism architectures can use to accordingly adjust the statistics.

Signed-off-by: Jan Beulich <jbeulich@novell.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>