Drill a hole big enough to then put a stack of washers over the shaft of the toggle bolt so you can fill the space between the cinderblock and the ouside of the drywall. then, when you tighten it down, you won't be trying to compress the drywall, and will be tightening it up on the washers. If you had some pipe, you could cut it and use it as a spacer as well. Or even a holesaw, and use wood. If the wall isn't straight, or the sink isn't flat, don't try to tighten it too much, or you'll crack the porcelain.