[IA64] Fix unexpected interrupt vector handling

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>