ppc: softfloat float implementation issues

Per bug #1841491, Richard Henderson (rth) said:
> The float test failure is part of a larger problem for target/powerpc in which all float 
> routines are implemented incorrectly. They are all implemented as double operations with
> rounding to float as a second step. Which not only produces incorrect exceptions, as in
> this case, but incorrect > numerical results from the double rounding.
> 
> This should probably be split to a separate bug...