RFA: Add a builtin define for when the floating point registers are not available

[Aldy Hernandez]

> You are right of course.  Further testing of the patch showed me that I 
> needed this anyway, so imagine if you will that I had submitted the 
> patch with an identical change to gcc/config/rs6000/linux.h.  :-)

How about you put it in rs6000_override_options, and only do it once?