Add an XFAILed test for native "llvm" modules

This functionality is completely broken

src/test/run-pass/native-llvm.rs

@@ -0,0 +1,10 @@
+// xfail-test
+
+native "llvm" mod llvm {
+    fn thesqrt(n: float) -> float = "sqrt.f64";
+}
+
+fn main() {
+    let s = llvm::thesqrt(4.0);
+    assert 1.9 < s && s < 2.1;
+}