Fix handling for spurious interrupts not being mapped to any IRQs.

Currently, spurious interrupts that are not mapped to any IRQs are
handled as IRQ 15 (== IA64_SPURIOUS_VECTOR). But it is not proper
because vector != irq. We need special handlings for such spurious
interrupts not being mapped to any IRQs.
Signed-off-by: Kenji Kaneshige <kaneshige.kenji@jp.fujitsu.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>