[libc] Add FILENO related macros to unistd.h. (#124688)

Closes #124637.

This is necessary to build LLVM with LLVM-libc and align the behavior
with other libc implementations.
ref: https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/unistd.h.html

Author: c8ef

libc/include/llvm-libc-macros/unistd-macros.h

@@ -5,4 +5,8 @@
 #include "linux/unistd-macros.h"
 #endif
 
+#define STDIN_FILENO 0
+#define STDOUT_FILENO 1
+#define STDERR_FILENO 2
+
 #endif // LLVM_LIBC_MACROS_UNISTD_MACROS_H