[libc] Add stubbed-out stdio and libdl functions

In order to fix link errors with libunwind, stub out these functions for
now.

src/libraries/libc/arch/amd64/crt/libdl.cpp

@@ -0,0 +1,7 @@
+
+extern "C" {
+    // Stub out these libdl functions for libunwind until
+    // we have a real libdl
+    int dladdr(const void *, void *) { return 0; }
+    int dl_iterate_phdr(void *, void *) { return 0; }
+}