Fix std_detect on targets without feature detection

library/stdarch/crates/std_detect/src/detect/macros.rs

@@ -1,3 +1,4 @@
+#[allow(unused)]
 macro_rules! features {
     (
       @TARGET: $target:ident;

library/stdarch/crates/std_detect/src/detect/mod.rs

@@ -55,9 +55,12 @@ cfg_if! {
     } else {
         // Unimplemented architecture:
         mod arch {
+            #[doc(hidden)]
             pub enum Feature {
                 Null
             }
+            #[doc(hidden)]
+            pub mod __is_feature_detected {}
         }
     }
 }