Auto merge of #30856 - mneumann:thread_local_extern, r=alexcrichton

This will correctly add the thread_local attribute to the external static variable ```errno```:

```rust
extern {
     #[thread_local]
     static errno: c_int;
}
```

Before this commit, the thread_local attribute is ignored. Fixes #30795.

Thanks @alexcrichton for pointing out the solution.

src/test/auxiliary/thread-local-extern-static.rs

@@ -0,0 +1,17 @@
+// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// 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.
+
+#![feature(thread_local)]
+#![feature(cfg_target_thread_local)]
+#![crate_type = "lib"]
+
+#[no_mangle]
+#[cfg_attr(target_thread_local, thread_local)]
+pub static FOO: u32 = 3;

src/test/run-pass/thread-local-extern-static.rs

@@ -0,0 +1,26 @@
+// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// 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.
+
+// ignore-windows
+// aux-build:thread-local-extern-static.rs
+
+#![feature(thread_local)]
+#![feature(cfg_target_thread_local)]
+
+extern crate thread_local_extern_static;
+
+extern {
+    #[cfg_attr(target_thread_local, thread_local)]
+    static FOO: u32;
+}
+
+fn main() {
+    assert_eq!(FOO, 3);
+}