torture: Add --kcsan argument to top-level kvm.sh script

Although the existing --kconfig argument can be used to run KCSAN for
an rcutorture test, it is not as straightforward as one might like:

	--kconfig "CONFIG_DEBUG_INFO=y CONFIG_KCSAN=y \
		   CONFIG_KCSAN_ASSUME_PLAIN_WRITES_ATOMIC=n \
		   CONFIG_KCSAN_REPORT_VALUE_CHANGE_ONLY=n \
		   CONFIG_KCSAN_REPORT_ONCE_IN_MS=100000 \
		   CONFIG_KCSAN_VERBOSE=y CONFIG_KCSAN_INTERRUPT_WATCHER=y"

This commit therefore adds a "--kcsan" argument that emulates the above
--kconfig command.  Note that if you specify a Kconfig option using
-kconfig that conflicts with one that --kcsan adds, you get whatever
the script and the build system decide to give you.

Cc: Marco Elver <elver@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>