References to unloaded CAFs due to stale static flag field

This issues is the first item described by @lolotp here: #16525 (comment 192087):

There are still references to CAFs in the unloaded object code, but these CAFs were put on the revertible caf list (which set the static link field to 3) and thus were ignored by GC. Because of that, during the next GC, checkUnload determined that these ObjectCode struct can be freed despite the references into those ObjectCodes still existing. Next GC cycle would trigger crash while GC is trying to evacuate the mentioned CAFs.