Add Progspace.objfile_for_address

Add Progspace.objfile_for_address

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>