s390/test_unwind: use timer instead of udelay

Instead of registering an external interrupt handler and relying on
the udelay implementation, simply use a timer to get into irq context.

Acked-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>