s390x: generate asm offsets for the lowcore

Add the s390x psw and lowcore structs and generate asm offsets for lowcore
fields. Add all lowcore fields defined in the current PoP (z13).

Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20170531123925.4547-2-david@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>