Use backend interface for return statements.

	* go-gcc.cc: #include "tree-iterator.h", "gimple.h", and "gogo.h".
	(class Bfunction): Define.
	(Gcc_backend::assignment_statement): Rename from assignment.
	Check for errors.
	(Gcc_backend::return_statement): New function.
	(tree_to_function): New function.
	* Make-lang.in (go/go-gcc.o): Depend on tree-iterator.h,
	$(GIMPLE_H), and $(GO_GOGO_H).

From-SVN: r171959