Add tests for non-object IR file

Add tests for non-object IR file

This patch adds linker, nm and ar tests for non-object IR file, including
archive with non-object IR member.

ld/

	PR ld/17878
	* Makefile.am (noinst_LTLIBRARIES): Add libldtestplug2.la and
	libldtestplug3.la.
	(libldtestplug2_la_SOURCES): New.
	(libldtestplug2_la_CFLAGS): Likewise.
	(libldtestplug2_la_LDFLAGS): Likewise.
	(libldtestplug3_la_SOURCES): New.
	(libldtestplug3_la_CFLAGS): Likewise.
	(libldtestplug3_la_LDFLAGS): Likewise.
	* Makefile.in: Regenerated.
	* testplug2.c: New file.
	* testplug3.c: Likewise.

ld/testsuite/

	PR ld/17878
	* ld-plugin/func.c: Add some comments.
	* ld-plugin/plugin-13.d: New file.
	* ld-plugin/plugin-14.d: Likewise.
	* ld-plugin/plugin-15.d: Likewise.
	* ld-plugin/plugin-16.d: Likewise.
	* ld-plugin/plugin-17.d: Likewise.
	* ld-plugin/plugin-18.d: Likewise.
	* ld-plugin/plugin-19.d: Likewise.
	* ld-plugin/plugin-20.d: Likewise.
	* ld-plugin/plugin-21.d: Likewise.
	* ld-plugin/plugin-22.d: Likewise.
	* ld-plugin/plugin-23.d: Likewise.
	* ld-plugin/plugin-24.d: Likewise.
	* ld-plugin/plugin-25.d: Likewise.
	* ld-plugin/plugin-26.d: Likewise.
	* ld-plugin/plugin.exp (plugin2_name): New.
	(plugin3_name): Likewise.
	(plugin2_path): Likewise.
	(plugin3_path): Likewise.
	(testsrcfiles): Likewise.
	(testsrcfiles_notext): Likewise.
	(plugin_tests): Add tests for non-object IR file.
	(plugin_lib_tests): Likewise.
	(plugin_extra_elf_tests): Likewise.
	(plugin_src_tests): New tests for non-object IR file.
	Run nm --plugin test.
	Run ar --plugin test.
	Run plugin_src_tests.