ModGC

exclude mmtk from jits workspace #414

Job	Run time
check (default, macos-latest)	5m 44s
check (default, ubuntu-latest)	6m 31s
check (mmtk, release, macos-latest)	8m 11s
check (mmtk, release, ubuntu-latest)	8m 44s
	29m 10s

Provide feedback