gc: prevent LTO from eliminating rb_gc_before_fork #12852

Job	Run time
ruby-bench (--warmup=1 --bench=1)	14m 40s
make (check, ubuntu-24.04, capi)	10m 26s
make (test-bundled-gems)	20m 54s
make (check, i686)	11m 32s
make (check, cppflags=-DVM_CHECK_MODE)	9m 6s
make (test-bundler-parallel, 50)	23m 31s
make (check, --disable-yjit)	6m 52s
make (check, --enable-shared --enable-load-relative)	7m 35s
make-ibm	0s
Ubuntu result	2s
	1h 44m 38s

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