s390x/tcg: make STFL store into the lowcore

Using virtual memory access is wrong and will soon include low-address
protection checks, which is to be bypassed for STFL.

STFL is a privileged instruction and using LowCore requires
!CONFIG_USER_ONLY, so add the ifdef and move the declaration to the
right place.

This was originally part of a bigger STFL(E) refactoring.

Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20170927170027.8539-4-david@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>
