std: Compile libbacktrace with -fvisibility=hidden

We don't want these symbols exported from the standard library, this is
just an internal implementation detail of the standard library
currently.

Closes #34984

src/liballoc_jemalloc/pthread_atfork_dummy.c

+// 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.
+//
+// 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.
+
 // See comments in build.rs for why this exists
 int pthread_atfork(void* prefork,
                    void* postfork_parent,

src/libstd/build.rs

@@ -87,8 +87,9 @@ fn build_libbacktrace(host: &str, target: &str) {
     let compiler = gcc::Config::new().get_compiler();
     // only msvc returns None for ar so unwrap is okay
     let ar = build_helper::cc2ar(compiler.path(), target).unwrap();
-    let cflags = compiler.args().iter().map(|s| s.to_str().unwrap())
-                         .collect::<Vec<_>>().join(" ");
+    let mut cflags = compiler.args().iter().map(|s| s.to_str().unwrap())
+                             .collect::<Vec<_>>().join(" ");
+    cflags.push_str(" -fvisibility=hidden");
     run(Command::new("sh")
                 .current_dir(&build_dir)
                 .arg(src_dir.join("configure").to_str().unwrap()