gccrs: intrinsics: Use lambdas for wrapping_<op> intrinsics

gcc/rust/ChangeLog:

	* backend/rust-compile-intrinsic.cc (wrapping_op_handler): Refactor to return
	an `std::function`.
	(wrapping_op_handler_inner): Rename.
	(wrapping_add_handler): Remove function.
	(wrapping_sub_handler): Likewise.
	(wrapping_mul_handler): Likewise.