target-arm: Use new softfloat min/max functions for VMAX, VMIN

Use the new softfloat min/max functions to implement the Neon VMAX
and VMIN instructions. This allows us to get the right behaviour
for NaN and negative zero.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
