torture: Improve diagnostic for KCSAN-incapable compilers

Using --kcsan when the compiler does not support KCSAN results in this:

:CONFIG_KCSAN=y: improperly set
:CONFIG_KCSAN_REPORT_ONCE_IN_MS=100000: improperly set
:CONFIG_KCSAN_VERBOSE=y: improperly set
:CONFIG_KCSAN_INTERRUPT_WATCHER=y: improperly set
Clean KCSAN run in /home/git/linux-rcu/tools/testing/selftests/rcutorture/res/2020.06.16-09.53.16

This is a bit obtuse, so this commit adds checks resulting in this:

:CONFIG_KCSAN=y: improperly set
:CONFIG_KCSAN_REPORT_ONCE_IN_MS=100000: improperly set
:CONFIG_KCSAN_VERBOSE=y: improperly set
:CONFIG_KCSAN_INTERRUPT_WATCHER=y: improperly set
Compiler or architecture does not support KCSAN!
Did you forget to switch your compiler with --kmake-arg CC=<cc-that-supports-kcsan>?
Suggested-by: Marco Elver <elver@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Acked-by: Marco Elver <elver@google.com>

tools/testing/selftests/rcutorture/bin/kvm-recheck.sh

@@ -44,7 +44,8 @@ do
 			then
 				echo QEMU killed
 			fi
-			configcheck.sh $i/.config $i/ConfigFragment
+			configcheck.sh $i/.config $i/ConfigFragment > $T 2>&1
+			cat $T
 			if test -r $i/Make.oldconfig.err
 			then
 				cat $i/Make.oldconfig.err
@@ -73,7 +74,11 @@ do
 	done
 	if test -f "$rd/kcsan.sum"
 	then
-		if test -s "$rd/kcsan.sum"
+		if grep -q CONFIG_KCSAN=y $T
+		then
+			echo "Compiler or architecture does not support KCSAN!"
+			echo Did you forget to switch your compiler with '--kmake-arg CC=<cc-that-supports-kcsan>'?
+		elif test -s "$rd/kcsan.sum"
 		then
 			echo KCSAN summary in $rd/kcsan.sum
 		else