ModGC

Fix RGENGC_CHECK_MODE without VM_CHECK_MODE #2655

Job	Run time
check (mmtk, release, macos-latest)	5m 32s
check (default, macos-latest)	3m 36s
check (default, ubuntu-latest)	3m 27s
check (mmtk, release, ubuntu-latest)	3m 32s
	16m 7s

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