ArmPkg/ArmGicDxe: Fix double GIC EIOR write per interrupt
This commit fixes a bug in the GIC v2 and v3 drivers where the GICC_EOIR
(End Of Interrupt Register) is written twice for a single interrupt.
GicV(2|3)IrqInterruptHandler() calls the Interrupt Handler and then
GicV(2|3)EndOfInterrupt() on exit:
although gInterrupt->EndOfInterrupt() can be expected to have already
been called by InterruptHandler() [which is the case for the primary
in-tree handler in TimerDxe]
The fix moves the EndOfInterrupt() call inside the else case for
unregistered/spurious interrupts. This removes a potential race
condition that might have lost interrupts.