Add test case for #[thread_local] on extern statics

Author: mneumann

src/test/auxiliary/thread-local-extern-static.rs

@@ -0,0 +1,2 @@
+#[thread_local]
+pub static FOO: u32 = 3;

src/test/run-pass/thread-local-extern-static.rs

@@ -0,0 +1,10 @@
+extern crate thread_local_extern_static;
+
+extern {
+    #[thread_local]
+    static FOO: u32;
+}
+
+fn main() {
+    assert_eq!(FOO, 3);
+}