target-i386: fix helper_fxtract() wrt softfloat

With softfloat it's not possible to play with the overflow of an
unsigned value to get the 0 case partially correct. Use a special case
for that. Using a division to generate an infinity is the easiest way
that works for both softfloat and softfloat-native.

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