target-arm: Don't overflow when calculating value for signed VABAL

In the VABAL instruction we take the absolute difference of two
values of size x and store it in a result of size 2x. This means
we have to be careful to calculate the absolute difference using
a wide enough type that we don't accidentally overflow.

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