ModGC

Turn back on sweep thread during GC.start #2868

Job	Run time
check (mmtk, release, macos-latest)	5m 7s
check (mmtk, release, ubuntu-latest)	5m 59s
check (default, macos-latest)	3m 30s
check (default, ubuntu-latest)	4m 18s
	18m 54s

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