[RFC, MIPS] Relax NaN rules

[Rich Fuhler]

> The easiest way to do that is to add a flag that either (a) stops the
> compiler emitting a .nan at all or (b) gets it to emit ".nan legacy"
> regardless of the actual encoding used.  It's really just a slight
> variation on (2), the difference being that we might be using 2008
> under the covers.
>

Hi Richard, we talked about (a.) originally - it was the design of the libraries. Joseph,
as I recollect, you raised language issues with requirements for compile-time constant
values for NaNs. Would you accept a non-constant NaN implementation in glibc? Basically, 
I would  envision __builtin_nan("") to be 0.0/0.0. Probably not a problem for C++ or most code.

-rich