rcutorture: Enable concurrent rcutorture runs

The rcutorture tests run by default range from using one CPU to using
sixteen of them.  Therefore, rcutorture testing could be sped up
significantly simply by running the kernels in parallel.  Building
them in parallel is not all that helpful: "make -j" is usually a
better bet.  So this commit takes a new "--cpus" argument that
specifies how many CPUs rcutorture is permitted to use for its
parallel runs.  The default of zero does sequential runs as before.

The bin-packing is minimal, and will be grossly suboptimal for
some configurations.  However, powers of two work reasonably well.

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