torture: Make --kcsan argument also create a summary

The KCSAN tool emits a great many warnings for current kernels, for
example, a one-hour run of the full set of rcutorture scenarios results
in no fewer than 3252 such warnings, many of which are duplicates
or are otherwise closely related.  This commit therefore introduces
a kcsan-collapse.sh script that maps these warnings down to a set of
function pairs (22 of them given the 3252 individual warnings), placing
the resulting list in decreasing order of frequency of occurrence into
a kcsan.sum file.  If any KCSAN warnings were produced, the pathname of
this file is emitted at the end of the summary of the rcutorture runs.

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