ModGC

Clear Monitor in MonitorMixin#freeze #2692

Job	Run time
check (default, ubuntu-latest)	4m 36s
check (mmtk, release, ubuntu-latest)	6m 20s
check (mmtk, release, macos-latest)	6m 53s
check (default, macos-latest)	4m 45s
	22m 34s

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