Fix some clippy warnings (#1277)

cranelift/codegen/src/context.rs

@@ -175,6 +175,8 @@ impl Context {
     ///
     /// The machine code is not relocated. Instead, any relocations are emitted into `relocs`.
     ///
+    /// # Safety
+    ///
     /// This function is unsafe since it does not perform bounds checking on the memory buffer,
     /// and it can't guarantee that the `mem` pointer is valid.
     ///