gccrs: Move function-related methods into base class Backend

gccrs: Move function-related methods into base class Backend

gcc/rust/ChangeLog:

	* rust-backend.h
	(Backend::function): Make non-virtual.
	(Backend::function_defer_statement): Likewise.
	(Backend::function_set_parameters): Likewise.
	(Backend::write_global_definitions): Likewise.
	(Backend::write_export_data): Likewise.

	(Gcc_backend::function): Remove.
	(Gcc_backend::function_defer_statement): Remove.
	(Gcc_backend::function_set_parameters): Remove.
	(Gcc_backend::write_global_definitions): Remove.
	(Gcc_backend::write_export_data): Remove.
	* rust-gcc.cc
	(Gcc_backend::function): Rename to ...
	(Backend::function): ... here.
	(Gcc_backend::function_defer_statement):
	Fix a qualified lookup of Backend::label and rename to ...
	(Backend::function_defer_statement): ... here.
	(Gcc_backend::function_set_parameters) Rename to ...
	(Backend::function_set_parameters): ... here.
	(Gcc_backend::write_global_definitions): Rename to ...
	(Backend::write_global_definitions): ... here.
	(Gcc_backend::write_export_data): Rename to ...
	(Backend::write_export_data): ... here.

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