hw/intc/arm_gic: Drop running_irq and last_active arrays

hw/intc/arm_gic: Drop running_irq and last_active arrays

The running_irq and last_active arrays represent state which
doesn't exist in a real hardware GIC. The only thing we use
them for is updating the running priority when an interrupt
is completed, but in fact we can use the active-priority
registers to do this. The running priority is always the
priority corresponding to the lowest set bit in the active
priority registers, because only one interrupt at any
particular priority can be active at once.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1438089748-5528-5-git-send-email-peter.maydell@linaro.org