ModGC

Fix concurrent_set.c when found garbage object #2699

Job	Run time
check (mmtk, release, macos-latest)	4m 50s
check (default, macos-latest)	5m 13s
check (mmtk, release, ubuntu-latest)	6h 5m 0s
check (default, ubuntu-latest)	4m 44s
	6h 19m 47s

Provide feedback