s390/traps: Use pr_emerg() instead of printk()

Use pr_emerg() instead of printk() in case of a stack overflow,
providing the emergency printk level. Also slightly adjust the
printed text for pr_emerg() and panic().

Reviewed-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>