Fix linkage1 test which fails due to --as-needed

It appears that the --as-needed flag to linkers will not pull in a dynamic library unless it satisfies a non weak undefined symbol. The linkage1 test was creating a dynamic library where it was only used for a weak-symbol as part of an executable, so the dynamic library was getting discarded.

This commit adds another symbol to the library which satisfies a strong undefined symbol, so the library is pulled in to resolve the weak reference.

src/test/auxiliary/linkage1.rs

@@ -10,3 +10,5 @@
 
 #[no_mangle]
 pub static foo: int = 3;
+
+pub fn bar() {}

src/test/run-pass/linkage1.rs

@@ -26,6 +26,13 @@ extern {
 }
 
 fn main() {
+    // It appears that the --as-needed flag to linkers will not pull in a dynamic
+    // library unless it satisfies a non weak undefined symbol. The 'other' crate
+    // is compiled as a dynamic library where it would only be used for a
+    // weak-symbol as part of an executable, so the dynamic library woudl be
+    // discarded. By adding and calling `other::bar`, we get around this problem.
+    other::bar();
+
     assert!(!foo.is_null());
     assert_eq!(unsafe { *foo }, 3);
     assert!(something_that_should_never_exist.is_null());