Use GCC LTO wrapper to get real symbols from LTO IR objects

GCC LTO wrapper is needed to extract real symbols from LTO IR objects.
This patch does the following:

1. Set up GCC LTO wrapper for each LTO IR object.
2. Run GCC LTO wrapper to get the real object.
3. Extract symbol info from the real object.
4. Cleanup afterwards.

bfd/

	PR binutils/25355
	* configure.ac (HAVE_EXECUTABLE_SUFFIX): New AC_DEFINE.
	(EXECUTABLE_SUFFIX): Likewise.
	* config.in: Regenerated.
	* configure: Likewise.
	* plugin.c (bfd_plugin_close_and_cleanup): Removed.
	(plugin_list_entry): Add all_symbols_read, cleanup_handler,
	gcc, lto_wrapper, resolution_file, resolution_option, gcc_env,
	real_bfd, real_nsyms, real_syms, lto_nsyms and lto_syms.
	(get_lto_wrapper): New.
	(setup_lto_wrapper_env): Likewise.
	(current_plugin): Likewise.
	(register_all_symbols_read): Likewise.
	(register_cleanup): Likewise.
	(get_symbols): Likewise.
	(add_input_file): Likewise.
	(bfd_plugin_close_and_cleanup): Likewise.
	(claim_file): Removed.
	(register_claim_file): Set current_plugin->claim_file.
	(add_symbols): Make a copy of LTO symbols.  Set lto_nsyms and
	lto_syms in current_plugin.
	(try_claim): Use current_plugin->claim_file.  Call LTO plugin
	all_symbols_read handler.  Copy real symbols to plugin_data.
	Call LTO plugin cleanup handler.  Clean up for LTO wrapper.
	(try_load_plugin): Don't reuse the previous plugin for LTO
	wrapper.  Set up GCC LTO wrapper if possible.  Don't set
	plugin_list_iter->claim_file.
	(bfd_plugin_canonicalize_symtab): Use real LTO symbols if
	possible.
	* plugin.h (plugin_data_struct): Add real_bfd, real_nsyms and
	real_syms.

ld/

	PR binutils/25355
	* testsuite/ld-plugin/lto.exp: Run PR binutils/25355 test.
	* testsuite/ld-plugin/pr25355.c: New file.
	* testsuite/ld-plugin/pr25355.d: Likewise.
	* testsuite/lib/ld-lib.exp (run_cc_link_tests): Support compile
	only dump.

bfd/plugin.h

@@ -33,6 +33,9 @@ typedef struct plugin_data_struct
 {
   int nsyms;
   const struct ld_plugin_symbol *syms;
+  bfd *real_bfd;
+  long real_nsyms;
+  asymbol **real_syms;
 }
 plugin_data_struct;