Join test thread to make assertion effective in sym.rs test case

src/test/ui/asm/sym.rs

@@ -76,5 +76,5 @@ fn main() {
     std::thread::spawn(|| {
         assert_eq!(static_addr!(S1), &S1 as *const u32);
         assert_eq!(static_tls_addr!(S2), &S2 as *const u32);
-    });
+    }).join().unwrap();
 }