Add a few missing types and constants to headers.

This (and other stuff still to come) prevented xfstests from building out
of the box.

Bug: http://b/27952303
Change-Id: Id38fd37386d43fc351c56dca18785a465e3e6145
diff --git a/libc/include/sys/stat.h b/libc/include/sys/stat.h
index 257ded0..86668f6 100644
--- a/libc/include/sys/stat.h
+++ b/libc/include/sys/stat.h
@@ -135,6 +135,12 @@
 #define DEFFILEMODE (S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH) /* 0666 */
 #endif
 
+#if defined(__USE_BSD) || defined(__USE_GNU)
+#define S_IREAD S_IRUSR
+#define S_IWRITE S_IWUSR
+#define S_IEXEC S_IXUSR
+#endif
+
 extern int chmod(const char*, mode_t);
 extern int fchmod(int, mode_t);
 extern int mkdir(const char*, mode_t);