[S390] incomplete stack traces.

show_stack() passes a pointer to the current stack frame to show_trace().
Because of tail call optimization the pointer doesn't point to the original
stack frame anymory and therefore traces are wrong. Don't pass the pointer
of the current stack frame to show_trace(). Instead let show_trace()
calculate the pointer on its own.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>