This adds a new objfile_for_address method to gdb.Progspace. This
makes it easy to find the objfile for a given address.
There's a related PR; and while this change would have been sufficient
for my original need, it's not clear to me whether I should close the
bug. Nevertheless I think it makes sense to at least mention it here.
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=19288 Reviewed-By: Eli Zaretskii <eliz@gnu.org>