target/arm: Macroize helper_gvec_{s,u}dot_idx_{b,h}

We're about to add more variations on this theme.
Accept the inner loop for the _h variants, rather
than keep it unrolled.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20210525010358.152808-66-richard.henderson@linaro.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>