gold: Avoid sharing Plugin_list::iterator

class Plugin_manager has

  // A pointer to the current plugin.  Used while loading plugins.
  Plugin_list::iterator current_;

The same iterator is shared by all threads. It is OK to use it to load
plugins since only one thread loads plugins.  Avoid sharing Plugin_list
iterator in all other cases.

	PR gold/26200
	* plugin.cc (Plugin_manager::claim_file): Don't share Plugin_list
	iterator.
	(Plugin_manager::all_symbols_read): Likewise.
	(Plugin_manager::cleanup): Likewise.