Get rid of underscore.c

[Kai Henningsen]

> I think of c++filt like addr2line and the other binutils as small 
> utilities that give access to a mechanism that would otherwize only be 
> available by running up GDB (outch).  I don't think someone should be 
> required to download/build GCC to get access to that utility.

That sounds as if *all* those utilities - not only c++filt -  really
should be in a separate directory (maybe "debugutils"), accessible to all
the usual suspects (gcc, binutils, gdb).  Together with any library parts.

And it sounds as if that directory should be handled similar to libiberty.