ModGC

Fix GCC optimization bug eliminating rb_gc_before_fork() #2393

Job	Run time
check (mmtk, release, ubuntu-latest)	11m 28s
check (default, ubuntu-latest)	8m 5s
check (default, macos-latest)	9m 25s
check (mmtk, release, macos-latest)	11m 42s
	40m 40s

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