s390/dumpstack: use pr_cont within show_stack and die

Use pr_cont instead of printk calls also within show_stack and
die in order to avoid extra line breaks.

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