s390x/tcg: Implement VECTOR GATHER ELEMENT

s390x/tcg: Implement VECTOR GATHER ELEMENT

Let's start with a more involved one, but it is the first in the list
of vector support instructions (introduced with the vector facility).

Good thing is, we need a lot of basic infrastructure for this. Reading
and writing vector elements as well as checking element validity.

All vector instruction related translation functions will reside in
translate_vx.inc.c, to be included in translate.c - similar to how
other architectures handle it.

While at it, directly add some documentation (which contains parts about
things added in follow-up patches, but splitting this up does not make
too much sense). Also add ES_* defines heavily used later.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20190307121539.12842-5-david@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>