re PR driver/42690 (Undefined reference errors with -flto -fuse-linker-plugin)

re PR driver/42690 (Undefined reference errors with -flto -fuse-linker-plugin)

	PR driver/42690
	* gcc.c (LINK_COMMAND_SPEC): Remove hard-coded pass-through plugin
	options, replace by call of pass-through-libs spec function to process
	link_gcc_c_sequence spec.
	(lto_libgcc_spec): Delete variable.
	(static_specs[]): Remove related entry.
	(static_spec_functions[]): Add new entry for pass-through-libs.
	(main): Don't generate deleted lto_libgcc_spec.
	(pass_through_libs_spec_func): New function to implement the new
	pass-through-libs spec function.
	* doc/invoke.texi (pass-through-libs): Document new spec function.

From-SVN: r167091