s390x: fix storing/loading fregs to right address

LARL is wrong here, as we specify an explicit address. Let's use LA
(we could also drop it and fixup all loads/stores, but this is easier).

Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20180824115059.1517-3-david@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Janosch Frank <frankja@linux.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>