Fix a pthread_t handle leak rust-lang#114610

(cherry picked from commit 42a7080)

Author: g0djan

library/std/src/sys/wasi/thread.rs

@@ -47,6 +47,7 @@ cfg_if::cfg_if! {
                     stack_size: libc::size_t,
                 ) -> ffi::c_int;
                 pub fn pthread_attr_destroy(attr: *mut pthread_attr_t) -> ffi::c_int;
+                pub fn pthread_detach(thread: pthread_t) -> ffi::c_int;
             }
         }
 
@@ -178,6 +179,17 @@ impl Thread {
     }
 }
 
+cfg_if::cfg_if! {
+    if #[cfg(target_feature = "atomics")] {
+        impl Drop for Thread {
+            fn drop(&mut self) {
+                let ret = unsafe { libc::pthread_detach(self.id) };
+                debug_assert_eq!(ret, 0);
+            }
+        }
+    }
+}
+
 pub fn available_parallelism() -> io::Result<NonZeroUsize> {
     unsupported()
 }