softfloat: fix default-NaN mode

When the default-NaN mode is enabled, it should return the default NaN
value, but it should anyway raise the invalid operation flag if one of
the operand is an sNaN.

I have checked that this behavior matches the ARM and SH4 manuals, as
well as real SH4 hardware.

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