Use backend interface for function types.

Use backend interface for function types.

Drop type_tree() functions for Float_type and Complex_type.
Don't define builtin functions until gogo is created.

	* go-gcc.cc (Gcc_backend::error_type): Implement.
	(Gcc_backend::string_type): Remove.
	(Gcc_backend::function_type): Change signature and implement.
	(Gcc_backend::struct_type): Change signature.
	(Gcc_backend::slice_type, Gcc_backend::map_type): Remove.
	(Gcc_backend::channel_type, Gcc_backend::interface_type): Remove.
	(Gcc_backend::pointer_type): Check for error.
	* Make-lang.in (go/types.o): Depend on go/gofrontend/backend.h.

From-SVN: r172932