s390x: smp: Loop if secondary cpu returns into cpu setup again

Up to now a secondary cpu could have returned from the function it was
executing and ending up somewhere in cstart64.S. This was mostly
circumvented by an endless loop in the function that it executed.

Let's add a loop to the end of the cpu setup, so we don't have to rely
on added loops in the tests.

Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Message-Id: <20200429143518.1360468-6-frankja@linux.ibm.com>
Signed-off-by: David Hildenbrand <david@redhat.com>