wire up makefile to run codegen tests and add one to start

src/test/codegen/hello.cc

@@ -0,0 +1,12 @@
+#include <stddef.h>
+
+struct slice {
+  char const *p;
+  size_t len;
+};
+
+extern "C"
+void test() {
+  struct slice s = { .p = "hello",
+                     .len = 5 };
+}

src/test/codegen/hello.rs

@@ -0,0 +1,4 @@
+#[no_mangle]
+fn test() {
+    let _x = "hello";
+}