Use a new "retxl" member of CPUS290XState to return the "eXtra Low" part
of a 128-bit value.  That said, this will get used when two independent
values need returning (e.g. quotient+remainder) as well.

At the same time, shuffle the elements of CPUS390XState to get this new
space from existing padding in the structure.

Signed-off-by: Richard Henderson <rth@twiddle.net>