Put parallel sweep impl behind --enable-parallel-sweep configure option #12915

Job	Run time
ruby-bench (--warmup=1 --bench=1 --excludes=shipit)	14m 14s
make (check, i686)	3m 13s
make (check, cppflags=-DVM_CHECK_MODE)	4m 45s
make (test-bundler-parallel, 50)	22m 30s
make (test-bundled-gems)	13m 36s
make (check, --disable-yjit)	4m 30s
make (check, ubuntu-24.04, capi)	4m 57s
make (check, --enable-shared --enable-load-relative)	4m 44s
make-ibm	0s
Ubuntu result	2s
	1h 12m 31s

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