[libc][posix] add readme

components/libc/compilers/readme.md

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+This folder provides uniformed header files crossing different compiler platforms, and supports basic standard C library functions, such as memory management and time management, etc.

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.