Don't add empty target features for target-cpu=native on macOS

compiler/rustc_codegen_llvm/src/llvm_util.rs

@@ -702,7 +702,9 @@ pub(crate) fn global_llvm_features(sess: &Session, only_base_features: bool) ->
 
                 features_string
             };
-            features.extend(features_string.split(',').map(String::from));
+            if !features_string.is_empty() {
+                features.extend(features_string.split(',').map(String::from));
+            }
         }
         Some(_) | None => {}
     };

tests/run-make/target-cpu-native/foo.rs

@@ -1 +1,9 @@
 fn main() {}
+
+// This forces explicit emission of a +neon target feature on targets
+// where it is implied by the target-cpu, like aarch64-apple-darwin.
+// This is a regression test for #153397.
+#[cfg(target_feature = "neon")]
+#[target_feature(enable = "neon")]
+#[unsafe(no_mangle)]
+pub fn with_neon() {}

tests/run-make/target-cpu-native/rmake.rs

@@ -8,7 +8,12 @@
 use run_make_support::{run, rustc};
 
 fn main() {
-    let out = rustc().input("foo.rs").arg("-Ctarget-cpu=native").run().stderr_utf8();
+    let out = rustc()
+        .input("foo.rs")
+        .arg("-Ctarget-cpu=native")
+        .arg("-Zverify-llvm-ir")
+        .run()
+        .stderr_utf8();
     run("foo");
     // There should be zero warnings emitted - the bug would cause "unknown CPU `native`"
     // to be printed out.