Revert "Disable OnStack used for spawn."
Revert "Disable OnStack used for spawn."

This reverts commit a932b2329cfc8cab5cd2815badc92eeb04bc9817.

Until we replace this mechanism, we're going to lock operations.

jsyeo/rubinius,jsyeo/rubinius,kachick/rubinius,kachick/rubinius,jsyeo/rubinius,jsyeo/rubinius,kachick/rubinius,kachick/rubinius,jsyeo/rubinius,jsyeo/rubinius,kachick/rubinius,kachick/rubinius,kachick/rubinius,jsyeo/rubinius,kachick/rubinius