GC finalization cleanup

[markples]

Pulled from #91029

• Add FinalizerStartSeg, FinalizerMaxSeg, and MaxSeg to make some of the code resilient to changes in the layout of the finalization queue. Also make WalkFReachableObjects resilient to layout changes.
• Add FreeListSeg as redundant entry for FreeList
  • The free list section is a segment; it just has its boundary stored differently. In [NOT READY] GC: Rearrange finalization queue to put freelist in middle #91029 it will be moved to the middle of the queue.
  • This provides a convenient audit point for all references to FreeList for [NOT READY] GC: Rearrange finalization queue to put freelist in middle #91029. The ones that are renamed here should be independent of the queue layout.
  • [NOT READY] GC: Rearrange finalization queue to put freelist in middle #91029 will remove FreeList when the other uses are updated.
• Add case for SegQueueLimit of last segment. Add helper UsedCount.
• Fix GetNumberFinalizableObjects to include critical finalizers, though this method isn't called anywhere today.
• Add const int INITIAL_FINALIZER_ARRAY_SIZE = 100;

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Issue Details

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Author:	markples
Assignees:	-
Labels:	area-GC-coreclr
Milestone:	-

[markples]

I'm still learning how to test one of the DATAS codepaths and diag codepaths, but otherwise I ran short stretches of reliability framework on wks and svr and CI is clean. I also verified that this doesn't improve the finalization benchmark performance in any way, which would have been quite surprising.

[markples]

/azp run runtime

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[markples]

Failures are networking and a timeout, and the set is different from a previous run.