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>