Choose the ragged stop strategy when the GC kind is determined, so that
we do so with respect to a single measurement of pending unavailable
bytes.
Also remove assert in heap_should_mark_while_stopping, as it can be
called after stopping too, when evacuation is enabled.
We don't need a pin bit: we just need to mark pinned objects before
evacuation starts. This way we can remove the stopping / marking race
so that we can always mark while stopping.
