target-arm: Set Invalid flag for NaN in float-to-int conversions

When we catch the special case of an input NaN in ARM float to int
helper functions, set the Invalid flag as well as returning the
correct result.

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