添加rtthread相关代码

riscv/rtthread/components/libc/compilers/musl/syscalls.c

@@ -0,0 +1,20 @@
+/*
+ * Copyright (c) 2006-2021, RT-Thread Development Team
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * Change Logs:
+ * Date           Author       Notes
+ * 2022-11-18     Meco Man     first version
+ */
+
+#include <rtthread.h>
+
+int *__errno_location(void)
+{
+    return _rt_errno();
+}
+int *___errno_location(void)
+{
+    return _rt_errno();
+}