std: Prevent print panics when using TLS

Currently if a print happens while a thread is being torn down it may cause a
panic if the LOCAL_STDOUT TLS slot has been destroyed by that point. This adds a
guard to check and prints to the process stdout if that's the case (as we do for
if the slot is already borrowed).

Closes #29488

src/test/run-pass/issue-29488.rs

@@ -0,0 +1,30 @@
+// Copyright 2015 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.
+
+use std::thread;
+
+struct Foo;
+
+impl Drop for Foo {
+    fn drop(&mut self) {
+        println!("test2");
+    }
+}
+
+thread_local!(static FOO: Foo = Foo);
+
+fn main() {
+    // Off the main thread due to #28129, be sure to initialize FOO first before
+    // calling `println!`
+    thread::spawn(|| {
+        FOO.with(|_| {});
+        println!("test1");
+    }).join().unwrap();
+}