ModGC

Set restart_heaps to false after sweep thread wakeup #2733

Job	Run time
check (mmtk, release, macos-latest)	7m 10s
check (default, ubuntu-latest)	4m 33s
check (default, macos-latest)	5m 23s
check (mmtk, release, ubuntu-latest)	6h 5m 0s
	6h 22m 6s

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