gc: prevent LTO from eliminating rb_gc_before_fork #9647

Job	Run time
cargo clippy	32s
cargo test	1m 10s
make (yjit-bindgen, To fix: use patch in logs, --with-gcc=clang-14 --enable-yjit=dev --enable-zji...	5m 38s
make (check, --enable-yjit=dev)	13m 41s
make (test-bundled-gems, --enable-yjit=dev)	25m 14s
make (check, RUSTC='rustc +1.58.0', 1.58.0)	9m 7s
make (check, --enable-yjit=dev, --yjit-call-threshold=1 --yjit-verify-ctx --yjit-code-gc, -T --yj...	16m 59s
YJIT Ubuntu result	2s
	1h 12m 23s

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