coccinelle: automatically switch some uses of memcpy() → mempcpy()

Inspired by #22520, let's add a coccinelle script that converts this
automatically.

coccinelle/mempcpy.cocci

@@ -0,0 +1,13 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+@@
+expression x, y, z;
+@@
+- memcpy(x, y, z);
+- x += z;
++ x = mempcpy(x, y, z);
+@@
+expression x, y, z;
+@@
+- memcpy_safe(x, y, z);
+- x += z;
++ x = mempcpy_safe(x, y, z);