s390/sclp: introduce sclp_emergency_printk()

Introduce sclp_emergency_printk() which can be used to emit a message
in emergency cases. sclp_emergency_printk() is only supposed to be
used in cases where it can be assumed that regular console device
drivers may not work anymore.
For example this may be the case for unrecoverable machine checks.

Reviewed-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>