创建，添加sdram

rt-thread/components/libc/compilers/common/extension/fcntl/README.md

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+Because of the history issue, flags in fcntl.h, such as O_CREAT, have difference types of value. Some OS use hex flags and others use octal flags.
+
+In terms of RT-Thread, Keil, IAR and MSVC use octal flags, which is located in the `tcntl/octal` folder; newlib uses hex flags; musl uses octal flags.
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