irq.h: remove padding from irq_desc on 64bits

Impact: reduce struct irq_desc size

struct irq_desc: reorder to remove padding on 64bits

shrinks irq_desc to 128 bytes which saves data space & cache lines

On a generic x86_64/SMP build this reduces the reported data size by
64k.

Signed-off-by: Richard Kennedy <richard@rsk.demon.co.uk>
Signed-off-by: Ingo Molnar <mingo@elte.hu>