torture: Create KCSAN summaries for torture.sh runs

Currently, each -kcsan run in a torture.sh group of runs has its own
kcsan.sum summary.  This works, but there is usually a lot of duplication
between the runs.  This commit therefore also creates an overall kcsan.sum
file for the entire torture.sh run, if there was at least one -kcsan run.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>