[sgen] Preclean mod union entries before the finishing pause

[sgen] Preclean mod union entries before the finishing pause

When the concurrent mark ends, we have another concurrent phase where we scan the cardtable trying to clean as many cards as possible. The way we achieve this is by first copying all the mod union cards (for each block/los_obj) to a separate temporary card_table. When scanning we will only scan these cards. Therefore we will first preclean these cards in the actual mod_union card_tables, followed by a full memory barrier (which assures us that, when scanning, we will observe the changes that led to the corresponding card marks) and then by a normal mod_union scan, which can remark some of these cards, without any races with the preclean phase.