powerpc: Rework /proc/interrupts

On a large machine I noticed the columns of /proc/interrupts failed to line up
with the header after CPU9. At sufficiently large numbers of CPUs it becomes
impossible to line up the CPU number with the counts.

While fixing this I noticed x86 has a number of updates that we may as well
pull in. On PowerPC we currently omit an interrupt completely if there is no
active handler, whereas on x86 it is printed if there is a non zero count.

The x86 code also spaces the first column correctly based on nr_irqs.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>