MEDIUM: thread: Turn the group mask in thread set into a group counter

MEDIUM: thread: Turn the group mask in thread set into a group counter

If we want to be able to have more than 64 thread groups, we can no
longer use thread group masks as long.
One remaining place where it is done is in struct thread_set. However,
it is not really used as a mask anywhere, all we want is a thread group
counter, so convert that mask to a counter.