From: Christoph Hellwig <hch@lst.de>

currently only x86_64 and ia64 don't use the generic irq_cpustat code
and both have to workaround it's brokenness for the non-default case.

x86_64 defines an empty irq_cpustat_t even if it doesn't need one and
ia64 adds CONFIG_IA64 ifdefs around all users.  What about this patch
instead to make __ARCH_IRQ_STAT useable?