Use a global lock for exit on unix.

library/std/src/sys/pal/unix/os.rs

@@ -758,7 +758,13 @@ pub fn home_dir() -> Option<PathBuf> {
 }
 
 pub fn exit(code: i32) -> ! {
-    unsafe { libc::exit(code as c_int) }
+    use crate::sync::OnceLock;
+    // https://github.com/rust-lang/rust/issues/126600
+    // Ensure that only one Rust thread calls exit.
+    // Technically not enough to ensure soundness, since other code directly calling
+    // libc::exit will still race with this.
+    static EXIT_ONCE: OnceLock<!> = OnceLock::new();
+    *EXIT_ONCE.get_or_init(|| unsafe { libc::exit(code as c_int) })
 }
 
 pub fn getpid() -> u32 {