Is it safe to use a laptop AC adapter with 220V power if the AC cable is not rated for 220V?

It should be fine.

Your laptop's power consumption is probably less than 100 W (unless it's a particularly power-hungry workstation or gaming machine), which at 220 V works out to about 0.45 A,¹ or way less than the 2.5 A the cable is rated for. In fact, since voltage and current are inversely proportional (for constant wattage), you're actually running less current through the cable than it was meant for. So current-wise, you're good.

As for voltage, as far as I know, the 125 V rating is merely an indicator of which mains system it was intended to be used with. The real concern here would be the insulation - if for some reason it's rated for less than 220 V, you'd be creating a fire and shock hazard. From what I've read about typical laptop power cords, the insulation should be good for at least 300 V, but you may want to inspect the cable carefully, the insulation's maximum voltage should be marked on it somewhere.

Nevertheless, if you want to be 100% sure, get a cable that's meant for use with 220 V mains systems.

¹ - a bit more, actually, since for alternating current, the amount of power usable by devices (called real power) is less than current × voltage (which is called apparent power), but the number is in the ballpark.