Implement STB_GNU_UNIQUE handling.

Some symbols have to be identified process-wide by their name.  This is
particularly important for some C++ features (e.g., class local static data
and static variables in inline functions).  This cannot completely be
implemented with ELF functionality so far.  The STB_GNU_UNIQUE binding
helps by ensuring the dynamic linker will always use the same definition for
all symbols with the same name and this binding.