Turn back on sweep thread during GC.start #9831

Job	Run time
cargo clippy	53s
make (yjit-bindgen, To fix: use patch in logs, --with-gcc=clang-14 --enable-yjit=dev --enable-zji...	5m 3s
cargo test	59s
make (check, RUSTC='rustc +1.58.0', 1.58.0)	4m 51s
make (test-bundled-gems, --enable-yjit=dev)	8m 33s
make (check, --enable-yjit=dev)	5m 49s
make (check, --enable-yjit=dev, --yjit-call-threshold=1 --yjit-verify-ctx --yjit-code-gc, -T --yj...	5m 28s
YJIT Ubuntu result	3s
	31m 39s

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