target/arm: Implement MVE saturating doubling multiply accumulates

target/arm: Implement MVE saturating doubling multiply accumulates

Implement the MVE saturating doubling multiply accumulate insns
VQDMLAH, VQRDMLAH, VQDMLASH and VQRDMLASH.  These perform a multiply,
double, add the accumulator shifted by the element size, possibly
round, saturate to twice the element size, then take the high half of
the result.  The *MLAH insns do vector * scalar + vector, and the
*MLASH insns do vector * vector + scalar.

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