[libiberty] Add ARG_UNUSED as a C++-friendly replacement for ATTRIBUTE_UNUSED

[DJ Delorie]

This implementation doesn't address K&R style argument lists, although
that's soon to be a moot point ;-)

I'd also prefer this definition to be right after the ATTRIBUTE_UNUSED
definition, rather than randomly added to the end of the file.

However, I'm not convinced we need this yet.  By the time we require a
C++ compiler, 3.4.0 will be old enough to require that version.