refperf: Label experiment-number column "Runs"

The experiment-number column is currently labeled "Threads", which is
misleading at best.  This commit therefore relabels it as "Runs", and
adjusts the scripts accordingly.

Cc: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>