s390/test_unwind: add kretprobe tests

Add tests to verify that s390 kretprobes maintain a correct stack
chain and ensure their proper function.

Reviewed-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Tobias Huschle <huschle@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>