PR25593, --as-needed breaks DT_NEEDED order with linker plugin

PR25593, --as-needed breaks DT_NEEDED order with linker plugin

This patch delays setting up DT_NEEDED dynamic tags until all object
files and libraries have been opened and their symbols processed,
rather than adding the tags while processing symbols.  Tags are
ordered according to the position of the associated library on the
command line and linker scripts.  It is still possible with
--as-needed libs that are mentioned more than once for tags to be
ordered according to which mention was needed.  For example with
"--as-needed a.so b.so c.so b.so" when b.so is not needed by a.so or
any other prior object file but is needed by c.so, the order of tags
will be "A C B".

bfd/
	PR 25593
	* elf-bfd.h (struct elf_link_hash_table): Rename "loaded" to
	"dyn_loaded".
	(bfd_elf_add_dt_needed_tag): Declare.
	* elf-strtab.c (_bfd_elf_strtab_restore): Handle NULL buf.
	* elflink.c (bfd_elf_add_dt_needed_tag): Make global and rename
	from elf_add_dt_needed_tag.  Remove soname and doit param.
	(elf_link_add_object_symbols): Don't use elf_add_dt_needed_tag
	to see whether as-needed lib is already loaded, use dyn_loaded
	list instead.  When saving and restoring around as-needed lib
	handle possibility that dynstr has not been initialised.  Don't
	add DT_NEEDED tags here.  Limit dyn_loaded list to dynamic libs.
	Mark libs loaded via DT_NEEDED entries of other libs with
	DYN_NO_NEEDED if they should not be mentioned in DT_NEEDED of
	the output.
	(elf_link_check_versioned_symbol): Remove now unneccesary
	DYNAMIC check when traversing dyn_loaded list.
ld/
	PR 25593
	* ldelf.c (ldelf_try_needed): Add DT_NEEDED lib to input_bfds.
	(ldelf_after_open): Save state of input_bfds list before loading
	DT_NEEDED libs.  Traverse input_bfds list adding DT_NEEDED tags.
	Restore input_bfds list.
	* testsuite/ld-cris/gotplt1.d: Adjust for changed .dynstr order.