Introduce MODULAR_GC_FN #278

Job	Run time
make (check, --enable-yjit=dev, --yjit-call-threshold=1 --yjit-verify-ctx --yjit-code-gc, -T --yj...	22m 11s
cargo test	44s
make (check, --enable-yjit, --yjit)	6m 53s
YJIT macOS Arm64 result	3s
	29m 51s

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