rcutorture: add support for s390

Add the required values to identify_qemu() and
identify_bootimage().

Signed-off-by: Sven Schnelle <[e-mail]>
Acked-by: Heiko Carstens <[e-mail]>
Signed-off-by: Willy Tarreau <[e-mail]>
Signed-off-by: Paul E. McKenney <[e-mail]>

tools/testing/selftests/rcutorture/bin/functions.sh

···
 		qemu-system-aarch64)
 			echo arch/arm64/boot/Image
 			;;
+		qemu-system-s390x)
+			echo arch/s390/boot/bzImage
+			;;
 		*)
 			echo vmlinux
 			;;
···
 	elif echo $u | grep -q aarch64
 	then
 		echo qemu-system-aarch64
+	elif echo $u | grep -q 'IBM S/390'
+	then
+		echo qemu-system-s390x
 	elif uname -a | grep -q ppc64
 	then
 		echo qemu-system-ppc64