bench: Put the spawn bench back on libgreen

This bench is meant to exercise libgreen, not libnative. It recently caused the
auto-linux-32-nopt-t bot to fail as no output was produced for an hour.
auto merge of #13136 : alexcrichton/rust/spawn-deadlock, r=brson

This bench is meant to exercise libgreen, not libnative. It recently caused the
auto-linux-32-nopt-t bot to fail as no output was produced for an hour.