Use backend interface for labels and goto statements.

Use backend interface for labels and goto statements.

	* go-gcc.c (class Blabel): Define.
	(Gcc_backend::make_expression): New function.
	(get_identifier_from_string): New function.
	(Gcc_backend::label): New function.
	(Gcc_backend::label_definition_statement): New function.
	(Gcc_backend::goto_statement): New function.
	(Gcc_backend::label_address): New function.
	(expression_to_tree): New function.
	* Make-lang.in (go/expressions.o): Depend on
	go/gofrontend/backend.h.
	(go/gogo.o): Likewise.

From-SVN: r171968