RFC: Make a standalone GNU V3 C++ demangler

[DJ Delorie]

> This patch provides a more accurate C++ demangler for libiberty to use,
> if certain requirements can be met (g++, <vector>). If it can't, the old
> one is used instead. Therefore, there will be no added dependency for gdb
> or binutils.

The build dependency is a little strange, though.  Libiberty has to go
before binutils, binutils before gcc, gcc before libstdc++, libstdc++
before demangler, and demangler before binutils and gdb.

We might as well just put the new demangler in libiberty and only
enable it when we can build it.