torture: Add --kcsan-kmake-arg to torture.sh for KCSAN

In 2020, running KCSAN often requires careful choice of compiler.
This commit therefore adds a --kcsan-kmake-arg parameter to torture.sh
to allow specifying (for example) "CC=clang" to the kernel build process
to correctly build a KCSAN-enabled kernel.

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