ModGC

Add rbs support to tool/rdoc-srcdir for make html #2708

Job	Run time
check (mmtk, release, macos-latest)	9m 9s
check (default, macos-latest)	6m 51s
check (mmtk, release, ubuntu-latest)	11m 54s
check (default, ubuntu-latest)	7m 54s
	35m 48s

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