gc: prevent LTO from eliminating rb_gc_before_fork #9646

Job	Run time
cargo test	56s
cargo clippy	55s
make (check, RUSTC='rustc +1.58.0', 1.58.0)	8m 57s
make (test-bundled-gems, --enable-yjit=dev)	23m 54s
make (check, --enable-yjit=dev)	14m 0s
make (check, --enable-yjit=dev, --yjit-call-threshold=1 --yjit-verify-ctx --yjit-code-gc, -T --yj...	15m 43s
make (yjit-bindgen, To fix: use patch in logs, --with-gcc=clang-14 --enable-yjit=dev --enable-zji...	5m 43s
YJIT Ubuntu result	2s
	1h 10m 10s

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