[libstdc++] Option to allow symbol versioning

[Benjamin Kosnik]

> > +## The symbol version here is a SWAG based on libgcc-std.ver.
> > +GCC_3.1 {
> 
> The symbol version tag defines an ABI.  Since you implement different
> functionality from libgcc, you should have your own tag.

Say GLIBCPP_3.1? And CXXABI_1 for libsupc++?

(BTW, Something I'd been meaning to ask: shouldn't libsupc++ be renamed 
to libcxa?)

> Perhaps I should have made this more clear by choosing LIBGCC_3.1
> elsewhere, but its too late now.
> 
> > +#    DW.ref.__gxx_personality_v0;   # I have no idea what this is.
> 
> This shouldn't be exported.  In fact, I believe it is marked .hidden
> so that it _cannot_ be exported.

Thanks!