target/arm: Implement MVE VCVT with specified rounding mode

Implement the MVE VCVT which converts from floating-point to integer
using a rounding mode specified by the instruction.  We implement
this similarly to the Neon equivalents, by passing the required
rounding mode as an extra integer parameter to the helper functions.

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