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

bfd/plugin.c

@@ -203,7 +203,7 @@ try_claim (bfd *abfd)
 }
 
 static int
-try_load_plugin (const char *pname, bfd *abfd)
+try_load_plugin (const char *pname, bfd *abfd, int *has_plugin_p)
 {
   void *plugin_handle;
   int tv_size = 4;
@@ -212,6 +212,8 @@ try_load_plugin (const char *pname, bfd *abfd)
   ld_plugin_onload onload;
   enum ld_plugin_status status;
 
+  *has_plugin_p = 0;
+
   plugin_handle = dlopen (pname, RTLD_NOW);
   if (!plugin_handle)
     {
@@ -244,25 +246,63 @@ try_load_plugin (const char *pname, bfd *abfd)
   if (status != LDPS_OK)
     goto err;
 
+  *has_plugin_p = 1;
+
+  abfd->plugin_format = bfd_plugin_no;
+
   if (!claim_file)
     goto err;
 
   if (!try_claim (abfd))
     goto err;
 
+  abfd->plugin_format = bfd_plugin_yes;
+
   return 1;
 
  err:
-  plugin_handle = NULL;
   return 0;
 }
 
+/* There may be plugin libraries in lib/bfd-plugins.  */
+
+static int has_plugin = -1;
+
+static const bfd_target *(*ld_plugin_object_p) (bfd *);
+
 static const char *plugin_name;
 
 void
 bfd_plugin_set_plugin (const char *p)
 {
   plugin_name = p;
+  has_plugin = p != NULL;
+}
+
+/* Return TRUE if a plugin library is used.  */
+
+bfd_boolean
+bfd_plugin_specified_p (void)
+{
+  return has_plugin > 0;
+}
+
+extern const bfd_target plugin_vec;
+
+/* Return TRUE if TARGET is a pointer to plugin_vec.  */
+
+bfd_boolean
+bfd_plugin_target_p (const bfd_target *target)
+{
+  return target == &plugin_vec;
+}
+
+/* Register OBJECT_P to be used by bfd_plugin_object_p.  */
+
+void
+register_ld_plugin_object_p (const bfd_target *(*object_p) (bfd *))
+{
+  ld_plugin_object_p = object_p;
 }
 
 static int
@@ -274,11 +314,14 @@ load_plugin (bfd *abfd)
   struct dirent *ent;
   int found = 0;
 
+  if (!has_plugin)
+    return found;
+
   if (plugin_name)
-    return try_load_plugin (plugin_name, abfd);
+    return try_load_plugin (plugin_name, abfd, &has_plugin);
 
   if (plugin_program_name == NULL)
-    return 0;
+    return found;
 
   plugin_dir = concat (BINDIR, "/../lib/bfd-plugins", NULL);
   p = make_relative_prefix (plugin_program_name,
@@ -295,10 +338,13 @@ load_plugin (bfd *abfd)
     {
       char *full_name;
       struct stat s;
+      int valid_plugin;
 
       full_name = concat (p, "/", ent->d_name, NULL);
       if (stat(full_name, &s) == 0 && S_ISREG (s.st_mode))
-	found = try_load_plugin (full_name, abfd);
+	found = try_load_plugin (full_name, abfd, &valid_plugin);
+      if (has_plugin <= 0)
+	has_plugin = valid_plugin;
       free (full_name);
       if (found)
 	break;
@@ -316,10 +362,13 @@ load_plugin (bfd *abfd)
 static const bfd_target *
 bfd_plugin_object_p (bfd *abfd)
 {
-  if (!load_plugin (abfd))
+  if (ld_plugin_object_p)
+    return ld_plugin_object_p (abfd);
+
+  if (abfd->plugin_format == bfd_plugin_uknown && !load_plugin (abfd))
     return NULL;
 
-  return abfd->xvec;
+  return abfd->plugin_format == bfd_plugin_yes ? abfd->xvec : NULL;
 }
 
 /* Copy any private info we understand from the input bfd