softfloat: Add float{16,32,64}_muladd_scalbn

We currently have a flag, float_muladd_halve_result, to scale
the result by 2**-1.  Extend this to handle arbitrary scaling.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
3 files changed