Unify add/sub helpers and add a parameter for rounding.
This will allow saturating non-rounding to reuse this code.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
[PMM: fixed accidental use of '=' rather than '+=' in do_sqrdmlah_s]
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20200815013145.539409-15-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>