get_symbols() plugin API tweak to support --start-lib/--end-lib.

get_symbols() plugin API tweak to support --start-lib/--end-lib.

Let the plugin know that a file is not being included in the link by
returning LDPS_NO_SYMS from get_symbols().

include/
	* plugin-api.h (enum ld_plugin_tag): Add LDPT_GET_SYMBOLS_V3.

gold/
	* plugin.h (Pluginobj::get_symbol_resolution_info): Add version
	parameter.
	* plugin.cc (get_symbols_v3): New function.
	(Plugin::load): Add LDPT_GET_SYMBOLS_V3.
	(Pluginobj::get_symbol_resolution_info): Return LDPS_NO_SYMS when using
	new version.