[Patch] Use __builtin_clz* for __lg

[Benjamin Kosnik]

> > I guess this might matter when __lg is called from a template. 
> >     
> I went through the various uses and all seem safe to me.

Me too. 

Matching non-template functions in an overload set are considered "most
specialized" in terms of overload resolution. Perhaps this is where
Joe's confusion comes from.

I think Paolo's original patch description was correctly worded,
although perhaps requiring a subtle reading.

-benjamin