softfloat: Rename float*_is_nan() functions to float*_is_quiet_nan()

The softfloat functions float*_is_nan() were badly misnamed,
because they return true only for quiet NaNs, not for all NaNs.
Rename them to float*_is_quiet_nan() to more accurately reflect
what they do.

This change was produced by:
 perl -p -i -e 's/_is_nan/_is_quiet_nan/g' $(git grep -l is_nan)
(with the results manually checked.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Nathan Froyd <froydnj@codesourcery.com>
Acked-by: Edgar E. Iglesias <edgar.iglesias@gmail.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
