Pull htab_eq_string from gcc libiberty

[Tom Tromey]

Richard> It does make me wonder why, in the days of git, we aren't using git
Richard> modules in both tools now for libiberty and include, then we wouldn't 
Richard> have sync problems.

I wouldn't mind.  Submodules are a bit of a pain, but not really worse
than what we're doing currently.

Unfortunately I don't think we could share the top level this way.
Maybe we could come up with some solution, like making symlinks or
something.

Also the binutils-gdb include directory has files not present in gcc.
We'd have to clean that up as well.

Tom