So far you've done a pretty impressive job of drilling right down the center of the bolt but I would try a smaller diameter bit first to drill all the way through then, if needed, increase the bit size. You might get lucky with the smaller or larger bit, and it will catch and spin the bolt out. As mentioned already, first use some good penetrant like Kroil or PB blaster and soak for a day. You can also use a sheet metal screw as an "Easy Out" type of tool since you don't need reverse turning to remove that bolt. If you have some good vice grip pliers you may still be able to grab that stump and remove it but clamp it on as hard as you can.