target/arm: Implement MVE VMAXA, VMINA

target/arm: Implement MVE VMAXA, VMINA

Implement the MVE VMAXA and VMINA insns, which take the absolute
value of the signed elements in the input vector and then accumulate
the unsigned max or min into the destination vector.

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