torture: Rename RCU_BUILDONLY to TORTURE_BUILDONLY

This commit makes the torture scripts a bit more RCU-independent by
changing RCU_BUILDONLY to TORTURE_BUILDONLY.  It also removes an
unnecessary export command.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>