gccrs: Move statement-related methods into base class Backend

gcc/rust/ChangeLog:

	* rust-backend.h
	(Backend::init_statement): Make non-virtual.
	(Backend::assignment_statement): Likewise.
	(Backend::return_statement): Likewise.
	(Backend::if_statement): Likewise.
	(Backend::loop_expression): Likewise.
	(Backend::exit_expression): Likewise.
	(Backend::compound_statement): Likewise.
	(Backend::statement_list): Likewise.
	(Backend::exception_handler_statement): Likewise.

	(Gcc_backend::init_statement): Remove.
	(Gcc_backend::assignment_statement): Remove.
	(Gcc_backend::return_statement): Remove.
	(Gcc_backend::if_statement): Remove.
	(Gcc_backend::compound_statement): Remove.
	(Gcc_backend::statement_list): Remove.
	(Gcc_backend::exception_handler_statement): Remove.
	(Gcc_backend::loop_expression): Remove.
	(Gcc_backend::exit_expression): Remove.
	* rust-gcc.cc
	(Gcc_backend::init_statement): Rename to ...
	(Backend::init_statement): ... here.
	(Gcc_backend::assignment_statement): Rename to ...
	(Backend::assignment_statement): ... here.
	(Gcc_backend::return_statement): Rename to ...
	(Backend::return_statement): ... here.
	(Gcc_backend::exception_handler_statement): Rename to ...
	(Backend::exception_handler_statement): ... here.
	(Gcc_backend::if_statement): Rename to ...
	(Backend::if_statement): ... here.
	(Gcc_backend::loop_expression): Rename to ...
	(Backend::loop_expression): ... here.
	(Gcc_backend::exit_expression): Rename to ...
	(Backend::exit_expression): ... here.
	(Gcc_backend::compound_statement): Rename to ...
	(Backend::compound_statement): ... here.
	(Gcc_backend::statement_list): Rename to ...
	(Backend::statement_list): ... here.

Signed-off-by: Owen Avery <powerboat9.gamer@gmail.com>