[S390] fix dump_stack vs. %p and (null)

The s390 implemenation of dump_stack uses %p to display stack content.
Since d97106ab53f812910a62d18afb9dbe882819c1ba (Make %p print '(null)'
for NULL pointers) this causes a strange output for dump_stack:

[...]
Process basename (pid: 8822, task: 00000000b2ece038, ksp: 00000000b24d7b38)
04000000b5685c00 00000000b24d7760 0000000000000002 (null)
00000000b24d7800 00000000b24d7778 00000000b24d7778 00000000001052fe
(null) 00000000b24d7b38 (null) 000000000000000a
000000000000000d (null) 00000000b24d7760 00000000b24d77d8
000000000051a7e8 00000000001052fe 00000000b24d7760 00000000b24d77b0
Call Trace:
[...]

This patch changes our dump_stack to use the appropriate %x format.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>