RFC: add completion for "set gnutarget"

[Tom Tromey]

>>>>> "Yao" == Tom Tromey <tromey@redhat.com> writes:

>>>>> "Yao" == Yao Qi <yao@codesourcery.com> writes:
Yao> I am wondering whether bfd_target_list returns an invariant.  If so,
Yao> we don't have to call bfd_target_list every time on completion.

Yao> Yeah, I think it is.

Yao> Here's a new version that implements this.

I'm checking this in now.

Tom