Get rid of underscore.c

[DJ Delorie]

> Code like this needs to be pushed into libiberty where GDB can also use 
> it.  At present GDB has its own equivalent of these operations :-(

I don't agree that libiberty is the right place for keeping the
details of assembler syntax of various platforms.  That sounds more
like a bfd thing.

If it were part of the demangler, though, I think that would be OK.
More likely, the app should *tell* the demangler which characters are
valid.  GCC, for example, has command line options that change the set
of valid characters.