powerpc: define __ARCH_IRQ_EXIT_IRQS_DISABLED

powerpc calls irq_exit() with local irqs disabled, therefore it
can define __ARCH_IRQ_EXIT_IRQS_DISABLED.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>