Use re-entrant locks on stdio

libc/mem/reallocarray.c

@@ -26,6 +26,7 @@
  * @param ptr may be NULL for malloc() behavior
  * @param nmemb may be 0 for free() behavior; shrinking is promised too
  * @return new address or NULL w/ errno and ptr is NOT free()'d
+ * @threadsafe
  */
 void *reallocarray(void *ptr, size_t nmemb, size_t itemsize) {
   size_t n;