PATCH: Support the new demangler written in C++

[DJ Delorie]

> Whatever we do, please don't require changes for other packages
> which happen to use demangler in the installed libiberty.a. The open
> source packages don't have a very good reputation of a backward
> compatible API, let alone ABI. I hope libiberty.a is not one of
> them.

If you're worried about backwards compatibility, fix the C demangler
that's in libiberty.  I'm not adding another whole toplevel directory
to libiberty, which means any work you do on this "new" demangler will
not be part of libiberty anyway.  If a package wants your new
demangler, they'll have to arrange to include it in the build anyway,
so they might as well ask for it by name.

I still stand by my original opinion, that adding yet another
demangler and letting the old ones rot is a Really Bad Idea.

And I'm not keen on the idea of libiberty.a functioning differently
based on some quirk of the build environment, either.