s390x/tcg: make STFL store into the lowcore

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 <[email protected]>
Message-Id: <20170927170027[email protected]>
Reviewed-by: Richard Henderson <[email protected]>
Reviewed-by: Thomas Huth <[email protected]>
Signed-off-by: Cornelia Huck <[email protected]>