[S390] cpu hotplug: on cpu start wait until being marked active

This is the same as fd8a7de1 "x86: cpu-hotplug: Prevent softirq wakeup
on wrong CPU".
Unlike on x86 this doesn't fix a bug on s390 since we do not have
threaded interrupt handlers. However we want to keep the same
initialization order like on x86. This should prevent bugs caused by
code which assumes (and relies on) the init order is the same on each
architecture.

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