Handle symbols defined/referenced only within IR

Handle symbols defined/referenced only within IR

The plugin is called to claim symbols in an archive element from
plugin_object_p.  But those symbols aren't needed to create output.
They are defined and referenced only within IR.  get_symbols should
return resolution based on IR symbol kinds.

	PR ld/20070
	* Makefile.am (noinst_LTLIBRARIES): Add libldtestplug4.la.
	(libldtestplug4_la_SOURCES): New.
	(libldtestplug4_la_CFLAGS): Likewise.
	(libldtestplug4_la_LDFLAGS): Likewise.
	* Makefile.in: Regenerated.
	* plugin.c (get_symbols): Return resolution based on IR symbol
	kinds for symbols defined/referenced only within IR.
	* testplug4.c: New file.
	* ld/testsuite/ld-plugin/pr20070.d: Likewise.
	* ld/testsuite/ld-plugin/pr20070a.c: Likewise.
	* ld/testsuite/ld-plugin/pr20070b.c: Likewise.
	* testsuite/ld-plugin/plugin.exp (plugin4_name): New.
	(plugin4_path): Likewise.
	Add a test for ld/20070.