RFC: Add STB_GNU_SECONDARY

[Roland McGrath]

> Currently we use weak undefined symbol, foo, to do
> 
> if (&foo != 0)
>  foo is defined.
> else
>  foo isn't defined.
> 
> We want is to define foo as a secondary symbol so that
> we can always use foo without checking.  If there is a primary
> one in a .o file and .so file, we will get the primary one,
> otherwise, we will use the secondary one.

Why not use a weak definition in the file where you make the call?