Add pub to all the codegen tests

Otherwise the test function is internalized and LLVM will most likely optimize
it out.

src/test/codegen/stack-alloc-string-slice.rs

@@ -9,6 +9,6 @@
 // except according to those terms.
 
 #[no_mangle]
-fn test() {
+pub fn test() {
     let _x = "hello";
 }