sys/newlib_syscalls_default: Add _gettimeofday_r stub

sys/newlib_syscalls_default/syscalls.c

@@ -500,11 +500,19 @@ int _kill(pid_t pid, int sig)
 #ifdef MODULE_XTIMER
 int _gettimeofday_r(struct _reent *r, struct timeval *restrict tp, void *restrict tzp)
 {
-    (void)tzp;
     (void) r;
+    (void) tzp;
     uint64_t now = xtimer_now_usec64();
     tp->tv_sec = div_u64_by_1000000(now);
     tp->tv_usec = now - (tp->tv_sec * US_PER_SEC);
     return 0;
 }
+#else
+int _gettimeofday_r(struct _reent *r, struct timeval *restrict tp, void *restrict tzp)
+{
+    (void) tp;
+    (void) tzp;
+    r->_errno = ENOSYS;
+    return -1;
+}
 #endif