x86: smarten /proc/interrupts output

Impact: change /proc/interrupts output ABI

With the number of interrupts on large systems growing, assumptions on
the width an interrupt number requires when converted to a decimal
string turn invalid. Therefore, calculate the maximum number of digits
dynamically.

Signed-off-by: Jan Beulich <jbeulich@novell.com>
LKML-Reference: <49B911EB.76E4.0078.0@novell.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>