Add missing target_feature to the list of well known cfg names

compiler/rustc_session/src/config.rs

@@ -1038,6 +1038,7 @@ fn fill_well_known_names(&mut self) {
             sym::target_has_atomic_load_store,
             sym::target_has_atomic,
             sym::target_has_atomic_equal_alignment,
+            sym::target_feature,
             sym::panic,
             sym::sanitize,
             sym::debug_assertions,
@@ -1081,6 +1082,10 @@ fn fill_well_known_values(&mut self) {
             .into_iter()
             .map(|sanitizer| Symbol::intern(sanitizer.as_str().unwrap()));
 
+        // Unknown possible values:
+        //  - `feature`
+        //  - `target_feature`
+
         // No-values
         for name in [
             sym::doc,