s390x: set initial stack pointer to stackptr, not stacktop

... was wrong from the beginning and remained unnoticed until we started
playing with dynamic memory allocation.

Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20180213162321.20522-4-david@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>