Introduce MODULAR_GC_FN #15

Job	Run time
make (zjit-test, --enable-yjit --enable-zjit=dev)	4m 11s
make (zjit-bindgen, To fix: use patch in logs, --enable-zjit=dev --with-gcc=clang-14, /usr/lib/ll...	5m 10s
make (test-all, --enable-zjit=dev, ../src/test/ruby/test_zjit.rb)	3m 45s
ZJIT Ubuntu result	4s
	13m 10s

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