s390x/tcg: Implement VECTOR GALOIS FIELD MULTIPLY SUM (AND ACCUMULATE)

s390x/tcg: Implement VECTOR GALOIS FIELD MULTIPLY SUM (AND ACCUMULATE)

A galois field multiplication in field 2 is like binary multiplication,
however instead of doing ordinary binary additions, xor's are performed.
So no carries are considered.

Implement all variants via helpers. s390_vec_sar() and s390_vec_shr()
will be reused later on.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: David Hildenbrand <david@redhat.com>