target/arm: Split out gen_gvec_fn_zz

target/arm: Split out gen_gvec_fn_zz

Model the new function on gen_gvec_fn2 in translate-a64.c, but
indicating which kind of register and in which order.  Since there
is only one user of do_vector2_z, fold it into do_mov_z.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20200815013145.539409-3-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>