s390x: compile sclp-ascii.c

We can reuse strlen and memcpy. __pa(x) is already defined in page.h
We don't have (and need) defines for error values and consuming sclp
irqs is not required, as we won't be passing control to some other OS.

Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20170522085702.17856-5-david@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>