selftests/ftrace: add s390 support for kprobe args tests

This is the s390 variant of commit 9855c462 ("selftests/ftrace:
Add ppc support for kprobe args tests").
Acked-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

tools/testing/selftests/ftrace/test.d/kprobe/kprobe_args_string.tc

@@ -22,6 +22,9 @@ ppc64*)
 ppc*)
   ARG1=%r3
 ;;
+s390*)
+  ARG1=%r2
+;;
 *)
   echo "Please implement other architecture here"
   exit_untested

tools/testing/selftests/ftrace/test.d/kprobe/kprobe_args_syntax.tc

@@ -32,6 +32,10 @@ ppc*)
   GOODREG=%r3
   BADREG=%msr
 ;;
+s390*)
+  GOODREG=%r2
+  BADREG=%s2
+;;
 *)
   echo "Please implement other architecture here"
   exit_untested