Serial: Use IsGCActiveMark instead of AutoModifyRestore in SerialHeap::do_collection

The `IsGCActiveMark` is used by other GCs except SerialGC. In `SerialHeap::do_collection`, the statement shown below implements the same feature as `IsGCActiveMark`. It is good to revise it so that all the GCs have the consistent implementation.

```
AutoModifyRestore<bool> temporarily(_is_gc_active, true);
```

This issue was firstly reported at https://mail.openjdk.org/pipermail/hotspot-gc-dev/2024-March/046686.html .