use freestanding headers

core/lib/stdlib.c

@@ -1,6 +1,6 @@
 #include <stdint.h>
-#include <stdlib.h>
-#include <string.h>
+#include <zinnia/stdlib.h>
+#include <zinnia/string.h>
 
 static void reverse(char* s) {
     int i, j;