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>