PR30343 infrastructure

Make ldemul_before_plugin_all_symbols_read more useful.

	* ldlang.c (lang_process): Move call to
	ldemul_before_plugin_all_symbols_read outside BFD_SUPPORTS_PLUGINS.
	Allow backends to add to gc_sym_list before handling entry sym.
	* ldelf.c (ldelf_before_plugin_all_symbols_read): Test
	lto_plugin_active.