[Libc][armlibc] Add STDIN_FILENO, STDOUT_FILENO and STDERR_FILENO to armlibc/sys/unistd.h.

components/libc/compilers/armlibc/sys/unistd.h

@@ -4,6 +4,11 @@
 #include <rtthread.h>
 
 #ifdef RT_USING_DFS
+
+#define STDIN_FILENO    0       /* standard input file descriptor */
+#define STDOUT_FILENO   1       /* standard output file descriptor */
+#define STDERR_FILENO   2       /* standard error file descriptor */
+
 #include <dfs_posix.h>
 #else
 #define _FREAD      0x0001  /* read enabled */