ModGC

Get rid of weighted average for heap_allocatable_bytes_expand #2744

Job	Run time
check (mmtk, release, macos-latest)	6m 29s
check (default, macos-latest)	4m 21s
check (mmtk, release, ubuntu-latest)	6h 5m 0s
check (default, ubuntu-latest)	4m 31s
	6h 20m 21s

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