cpu/avr8_common: Add dummy thread_arch.h header

The thread_yield_higher function is complex enough to not inline it for
the avr8 cpu

cpu/avr8_common/include/thread_arch.h

@@ -0,0 +1,33 @@
+/*
+ * Copyright (C) 2021 Koen Zandberg <koen@bergzand.net>
+ *               2021 Inria
+ *
+ * This file is subject to the terms and conditions of the GNU Lesser General
+ * Public License v2.1. See the file LICENSE in the top level directory for more
+ * details.
+ */
+
+/**
+ * @ingroup     cpu_avr8_common
+ * @{
+ *
+ * @file
+ * @brief       Implementation of the kernels thread interface
+ *
+ * @author      Koen Zandberg <koen@bergzand.net>
+ *
+ * @}
+ */
+#ifndef THREAD_ARCH_H
+#define THREAD_ARCH_H
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* THREAD_ARCH_H */
+/** @} */