Fix a file descriptor leak in gold.

Fix a file descriptor leak in gold.

When an LTO linker plugin claims an external member of a thin archive, gold
does not properly unlock the file and make its file descriptor available for
reuse. This patch fixes the problem by modifying Archive::include_member to
unlock the object file via an RAII class instance, ensuring that it will be
unlocked no matter what path is taken through the function.

gold/
	PR gold/15660
	* archive.cc (Thin_archive_object_unlocker): New class.
	(Archive::include_member): Unlock external members of thin archives.
	* testsuite/Makefile.am (plugin_test_1): Rename .syms files.
	(plugin_test_2): Likewise.
	(plugin_test_3): Likewise.
	(plugin_test_4): Likewise.
	(plugin_test_5): Likewise.
	(plugin_test_6): Likewise.
	(plugin_test_7): Likewise.
	(plugin_test_8): Likewise.
	(plugin_test_9): Likewise.
	(plugin_test_10): Likewise.
	(plugin_test_11): New test case.
	* testsuite/Makefile.in: Regenerate.
	* testsuite/plugin_test.c (claim_file_hook): Check for parallel .syms
	file to decide whether to claim file.
	(all_symbols_read_hook): Likewise.
	* testsuite/plugin_test_1.sh: Adjust expected output.
	* testsuite/plugin_test_2.sh: Likewise.
	* testsuite/plugin_test_3.sh: Likewise.
	* testsuite/plugin_test_6.sh: Likewise.
	* testsuite/plugin_test_tls.sh: Likewise.
	* testsuite/plugin_test_11.sh: New testcase.