Sidestep link error from rustfix'ed code by using a *defined* static.

As a drive-by, added `-g` to the compile-flags so that the test more
reliably fails to compile when the extern static in question is *not*
provided. (I.e. this is making the test more robust in the face of
potential future revisions.)

Fix #54388.

src/test/ui/extern/extern-const.fixed

@@ -0,0 +1,25 @@
+// This test is checking that extern items cannot be const.  It also
+// checks that `rustfix` suggests the alternative of using an extern
+// static.
+//
+// #54388: an unused reference to an undefined static may or may not
+// compile. To sidestep this by using one that *is* defined.
+
+// run-rustfix
+// compile-flags: -g -Z continue-parse-after-error
+#![feature(libc)]
+extern crate libc;
+
+#[link(name = "rust_test_helpers", kind = "static")]
+extern "C" {
+    static rust_dbg_static_mut: libc::c_int; //~ ERROR extern items cannot be `const`
+}
+
+fn main() {
+    // We suggest turning the (illegal) extern `const` into an extern `static`,
+    // but this also requires `unsafe` (a deny-by-default lint at comment time,
+    // future error; Issue #36247)
+    unsafe {
+        let _x = rust_dbg_static_mut;
+    }
+}

src/test/ui/extern/extern-const.rs

@@ -1,25 +1,25 @@
-// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
-// file at the top-level directory of this distribution and at
-// http://rust-lang.org/COPYRIGHT.
+// This test is checking that extern items cannot be const.  It also
+// checks that `rustfix` suggests the alternative of using an extern
+// static.
 //
-// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
-// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
-// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
-// option. This file may not be copied, modified, or distributed
-// except according to those terms.
+// #54388: an unused reference to an undefined static may or may not
+// compile. To sidestep this by using one that *is* defined.
 
-// FIXME(#54388): re-enable rustfix later, when this test has consistent output across targets
-// compile-flags: -Z continue-parse-after-error
+// run-rustfix
+// compile-flags: -g -Z continue-parse-after-error
+#![feature(libc)]
+extern crate libc;
 
+#[link(name = "rust_test_helpers", kind = "static")]
 extern "C" {
-    const C: u8; //~ ERROR extern items cannot be `const`
+    const rust_dbg_static_mut: libc::c_int; //~ ERROR extern items cannot be `const`
 }
 
 fn main() {
     // We suggest turning the (illegal) extern `const` into an extern `static`,
     // but this also requires `unsafe` (a deny-by-default lint at comment time,
     // future error; Issue #36247)
     unsafe {
-        let _x = C;
+        let _x = rust_dbg_static_mut;
     }
 }

src/test/ui/extern/extern-const.stderr

@@ -1,7 +1,7 @@
 error: extern items cannot be `const`
   --> $DIR/extern-const.rs:15:5
    |
-LL |     const C: u8; //~ ERROR extern items cannot be `const`
+LL |     const rust_dbg_static_mut: libc::c_int; //~ ERROR extern items cannot be `const`
    |     ^^^^^ help: try using a static value: `static`
 
 error: aborting due to previous error