Expose ftello64.

Also add the missing test that would have caught this.

Bug: http://b/24807045
Change-Id: I756a4d825595b52396b15898e3f717156fd1ba2f
diff --git a/tests/stdio_test.cpp b/tests/stdio_test.cpp
index b6f6526..0d21dec 100644
--- a/tests/stdio_test.cpp
+++ b/tests/stdio_test.cpp
@@ -1165,6 +1165,7 @@
 static void AssertFileOffsetAt(FILE* fp, off64_t offset) {
   EXPECT_EQ(offset, ftell(fp));
   EXPECT_EQ(offset, ftello(fp));
+  EXPECT_EQ(offset, ftello64(fp));
   fpos_t pos;
   fpos64_t pos64;
   EXPECT_EQ(0, fgetpos(fp, &pos));