添加rtthread相关代码

riscv/rtthread/components/libc/posix/readme.md

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+This folder provides functions that are not part of the standard C library but are part of the POSIX.1 (IEEE Standard 1003.1) standard.
+
+
+
+## NOTE
+
+1. For consistency of compilation results across the different of platforms(gcc, keil, iar) , use:
+   - `#include <sys/time.h>` to instead of `#include <time.h>`
+   - `#include <sys/errno.h>` to instead of `#include <errno.h>`
+   - `#include <sys/signal.h>` to instead of `#include <signal.h>`
+