Impact: cleanup

Arches that have their own irq_regs definition are expected to
define ARCH_HAS_OWN_IRQ_REGS or else a generic (unused) set
will also be defined in lib/irq_regs.c

Sparse noticed the unused generic one had no prototype:
lib/irq_regs.c:15:1: warning: symbol 'per_cpu____irq_regs' was not declared. Should it be static?
Signed-off-by: NHarvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>