[Patch] Use __builtin_clz* for __lg

[Paolo Carlini]

Joe Buck wrote:
> Your "specializations" aren't specializations, but ordinary functions.
> I'm not sure whether this matters in practice, though.
>
> That is, you define __lg(int), not __lg<int>(int).
>   
Right. In fact I carefully used the term -overloads- in the ChangeLog.
> I guess this might matter when __lg is called from a template. 
>   
I went through the various uses and all seem safe to me.

Thanks,
Paolo.