add fortified readlink/readlinkat implementations

Change-Id: Ia4b1824d20cad3a072b9162047492dade8576779
diff --git a/libc/bionic/readlink.cpp b/libc/bionic/readlink.cpp
index 3bb7bc1..a53f933 100644
--- a/libc/bionic/readlink.cpp
+++ b/libc/bionic/readlink.cpp
@@ -26,6 +26,8 @@
  * SUCH DAMAGE.
  */
 
+#undef _FORTIFY_SOURCE
+
 #include <fcntl.h>
 #include <sys/stat.h>
 #include <sys/types.h>