Introduce MODULAR_GC_FN #667

Job	Run time
make (check, cppflags=-DVM_CHECK_MODE)	8m 1s
make (check, i686)	9m 35s
make (check, --disable-yjit)	6m 51s
make (check, ubuntu-24.04)	8m 19s
make (check, ubuntu-24.04-arm)	7m 23s
make (test-bundler-parallel, 50)	27m 55s
make (check, --enable-shared --enable-load-relative)	7m 7s
make (test-bundled-gems)	17m 41s
Ubuntu result	2s
	1h 32m 54s

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