Forbid unsafe_op_in_unsafe_fn in hurd-specific os and sys files

Adding it did not cause any error. Most of this falls back on Unix already.

See rust-lang#127747

std/src/os/hurd/mod.rs

@@ -1,6 +1,7 @@
 //! Hurd-specific definitions
 
 #![stable(feature = "raw_ext", since = "1.1.0")]
+#![forbid(unsafe_op_in_unsafe_fn)]
 
 pub mod fs;
 pub mod raw;