Add <libc-pointer-arith.h> header for simplifying backports.

diff --git a/include/libc-pointer-arith.h b/include/libc-pointer-arith.h
new file mode 100644
index 0000000000000000..775374c9b8b31bcc
--- /dev/null
+++ b/include/libc-pointer-arith.h
@@ -0,0 +1,2 @@
+/* Compatibility header to simplify backports.  */
+#include <libc-internal.h>