Turn back on sweep thread during GC.start #13038

Job	Run time
ruby-bench (--warmup=1 --bench=1 --excludes=shipit)	13m 50s
make (check, i686)	5m 8s
make (check, --disable-yjit)	5m 20s
make (check, cppflags=-DVM_CHECK_MODE)	5m 3s
make (check, --enable-shared --enable-load-relative)	5m 12s
make (check, ubuntu-24.04, capi)	5m 23s
make (test-bundler-parallel, 50)	22m 0s
make (test-bundled-gems)	13m 15s
make-ibm	-1s
Ubuntu result	3s
	1h 15m 13s

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