ModGC

Use lock-free stack for swept_pages #2713

Job	Run time
check (default, macos-latest)	4m 13s
check (mmtk, release, ubuntu-latest)	6h 5m 0s
check (mmtk, release, macos-latest)	5m 56s
check (default, ubuntu-latest)	4m 24s
	6h 19m 33s

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