Merge linker plugin handling into BFD plugin support

Merge linker plugin handling into BFD plugin support

Linker plugin_maybe_claim is the interface of linker plugin support.
This patch extracts linker plugin_maybe_claim into plugin_object_p and
makes it available to BFD via a new function:

void register_ld_plugin_object_p (const bfd_target *(*) (bfd *));

bfd_plugin_object_p calls plugin_object_p registered by linker first.  It
adds an enum bfd_plugin_format field and a pointer to plugin dummy BFD so
that plugin_object_p stores plugin dummy BFD to allow plugin_maybe_claim
to retrieve it later.

bfd/

	PR ld/17878
	* bfd.c (bfd_plugin_format): New.
	(bfd): Add plugin_format and plugin_dummy_bfd.
	* plugin.c (try_load_plugin): Take a pointer to bfd_boolean
	argument to return TRUE if any plugin is found.  Set plugin_format.
	(has_plugin): New.
	(bfd_plugin_target_p): New.
	(bfd_plugin_specified_p): Likewise.
	(bfd_plugin_target_p): Likewise.
	(register_ld_plugin_object_p): Likewise.
	(bfd_plugin_set_plugin): Set has_plugin.
	(load_plugin): Cache try_load_plugin result.
	(bfd_plugin_object_p): Try ld_plugin_object_p first.  Check
	plugin_format.
	* plugin.h (bfd_plugin_target_p): New.
	(bfd_plugin_specified_p): Likewise.
	(register_ld_plugin_object_p): Likewise.
	* bfd-in2.h: Regenerated.

ld/

	PR ld/17878
	* plugin.c: Include ../bfd/plugin.h.
	(plugin_get_ir_dummy_bfd): Call bfd_create with
	link_info.output_bfd instead of srctemplate.  Copy BFD info
	from srctemplate only if it doesn't use BFD plugin target
	vector.
	(plugin_load_plugins): Call register_ld_plugin_object_p with
	(plugin_object_p)
	(plugin_maybe_claim): Renamed to ...
	(plugin_object_p): This.  Return dummy BFD target vector if
	input is calimed by plugin library, otherwise return NULL.
	Update plugin_format and plugin_dummy_bfd.
	(plugin_maybe_claim): New.  Use plugin_object_p.

xx