s390/smp: Use monotonic clock in smp_emergency_stop()

This is a cosmetic change because when in smp_emergency_stop()
the system is going to die anyway. But still change the code
to use get_tod_clock_monotonic() to prevent people from copying
broken code.

Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>