ModGC

Use lock-free stack for swept_pages #2714

Job	Run time
check (mmtk, release, macos-latest)	6m 15s
check (default, ubuntu-latest)	4m 16s
check (default, macos-latest)	4m 35s
check (mmtk, release, ubuntu-latest)	6h 5m 0s
	6h 20m 6s

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