添加rtthread相关代码

riscv/rtthread/components/libc/compilers/common/include/unistd.h

@@ -0,0 +1,32 @@
+/*
+ * Copyright (c) 2006-2023, RT-Thread Development Team
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * Change Logs:
+ * Date           Author       Notes
+ * 2023/06/08     Bernard      Add macro definition for `#pragma once`
+ */
+
+#ifndef UNISTD_H__
+#define UNISTD_H__
+
+#include "sys/unistd.h"
+
+#ifndef F_OK
+#define	F_OK	0
+#endif
+
+#ifndef R_OK
+#define	R_OK	4
+#endif
+
+#ifndef W_OK
+#define	W_OK	2
+#endif
+
+#ifndef X_OK
+#define	X_OK	1
+#endif
+
+#endif /* UNISTD_H__ */