ModGC

Change direction in gc_sweep_continue if sweep thread didn't make fg … #2717

Job	Run time
check (mmtk, release, ubuntu-latest)	6h 5m 1s
check (mmtk, release, macos-latest)	7m 8s
check (default, macos-latest)	4m 21s
check (default, ubuntu-latest)	4m 46s
	6h 21m 16s

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