Implement scandirat and scandirat64.

Bug: http://b/12612339
Change-Id: Id3b249a884fe08964b26a017ae9574961f0cb441
diff --git a/libc/libc.map b/libc/libc.map
index fbceb83..bb67b5c 100644
--- a/libc/libc.map
+++ b/libc/libc.map
@@ -1346,6 +1346,8 @@
     preadv64;
     pwritev;
     pwritev64;
+    scandirat;
+    scandirat64;
     strchrnul;
 } LIBC;