target-arm: Handle UNDEF cases for Neon VLD/VST multiple-structures

Correctly UNDEF for Neon VLD/VST "multiple structures" forms where the
align field is not valid.

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