s390x: Clear first stack frame and end backtrace early

When setting the first stack frame to 0, we can check for a 0
backchain pointer when doing backtraces to know when to stop.

Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
Reviewed-by: Nico Boehr <nrb@linux.ibm.com>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Link: https://lore.kernel.org/r/20230112154548.163021-7-frankja@linux.ibm.com
Message-Id: <20230112154548.163021-7-frankja@linux.ibm.com>