Introduce MODULAR_GC_FN #571

Job	Run time
cargo clippy	32s
cargo test	56s
make (yjit-bindgen, To fix: use patch in logs, --with-gcc=clang-14 --enable-yjit=dev, /usr/lib/ll...	5m 6s
make (check, RUSTC='rustc +1.58.0', 1.58.0)	8m 47s
make (test-bundled-gems, --enable-yjit=dev)	24m 6s
make (check, --enable-yjit=dev, --yjit-call-threshold=1 --yjit-verify-ctx --yjit-code-gc, -T --yj...	14m 57s
make (yjit-bench, --enable-yjit=dev, --yjit-stats, true)	6m 15s
make (check, --enable-yjit=dev)	11m 29s
YJIT Ubuntu result	3s
	1h 12m 11s

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