Add --debug=plugin option to record plugin actions.

Add --debug=plugin option to record plugin actions.

With --debug=plugin, gold will log all plugin actions to a log file,
and make copies of plugin-supplied replacement files in a temporary
directory created under the current working directory.

gold/
	* debug.h (DEBUG_PLUGIN): New constant.
	(DEBUG_ALL): Add DEBUG_PLUGIN.
	(debug_string_to_enum): Likewise.
	* plugin.cc (make_sized_plugin_object): Add filename parameter.
	(Plugin_recorder): New class.
	(Plugin_manager::~Plugin_manager): Delete recorder_.
	(Plugin_manager::load_plugins): Create and initialize recorder_.
	(Plugin_manager::claim_file): Record claimed and unclaimed files.
	(Plugin_manager::make_plugin_object): Use object name as name for
	plugin object, if available.
	(Plugin_manager::add_input_file): Record replacement files.
	(Sized_pluginobj::do_add_symbols): Record plugin symbols.
	(Plugin_finish::run): Call Plugin_recorder::finish().
	(make_sized_plugin_object): Add filename parameter and pass to
	Sized_pluginobj constructor.
	* plugin.h (Plugin::filename): New method.
	(Plugin::recorder): New method.
	(Plugin::recorder_): New data member.