Add an annotate attribute to this test.

This makes it generate an llvm.global.annotation, which the backend
should ignore.

Author: sunfishcode

tests/core/test_llvm_used.in

@@ -2,6 +2,7 @@
 #include <emscripten.h>
 
 extern "C" {
+        __attribute__((annotate("hello attribute world")))
         EMSCRIPTEN_KEEPALIVE void foobar(int x) {
                 printf("Worked! %d\n", x);
         }