[sgen] Only do fence when concurrent collection is running.

[sgen] Only do fence when concurrent collection is running.

The store/store fence incurs quite a high cost, so we only want it to
run when absolutely necessary.  The most efficient way is to
separately compile nursery object operations for when the concurrent
collector is running, so there has to be no dynamic check.

In the future we might want to try batching updates of references to
moved objects so we have to do only one fence per batch.