XCOFF: use bfd_coff_close_and_cleanup

Free memory on closing bfds.  The COFF close_and_cleanup does more
work than _bfd_generic_close_and_cleanup (defined as
_bfd_archive_close_and_cleanup).

	* coff-rs6000.c (_bfd_xcoff_close_and_cleanup): Define as
	_bfd_coff_close_and_cleanup.
	* coff64-rs6000.c (rs6000_xcoff64_vec, rs6000_xcoff64_aix_vec): Use
	_bfd_coff_close_and_cleanup.