fpu/softfloat: Make is_nan et al available to softfloat-specialize.h

fpu/softfloat: Make is_nan et al available to softfloat-specialize.h

We will need these helpers within softfloat-specialize.h, so move
the definitions above the include.  After specialization, they will
not always be used so mark them to avoid the Werror.

Tested-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>