bfd_close and target free_cached_memory

bfd_close and target free_cached_memory

bfd_free_cached_info is used in just one place in archive.c, which
means most times we reach bfd_close the function isn't called.  On the
other hand, if bfd_free_cached_info is called we can't do much on the
bfd since it loses all its obj_alloc memory.  This restricts what can
be done in a target _close_and_cleanup.  In particular you can't look
at sections, which leads to duplication of code in target
close_and_cleanup and free_cached_info, eg. elfnn-aarch64.c.

	* opncls.c (_bfd_delete_bfd): Call bfd_free_cached_info.
	* elfnn-aarch64.c (elfNN_aarch64_close_and_cleanup): Delete.
	(bfd_elfNN_close_and_cleanup): Don't define.
	* som.c (som_bfd_free_cached_info): Don't call
	_bfd_generic_close_and_cleanup here.
	(som_close_and_cleanup): Define as _bfd_generic_close_and_cleanup.