Add mempcpy(3)

We'll use it for implementing stpecpy(), and may be interesting to have
it around.

Signed-off-by: Alejandro Colomar <alx@kernel.org>

lib/mempcpy.h

@@ -0,0 +1,31 @@
+/*
+ * SPDX-FileCopyrightText:  2023, Alejandro Colomar <alx@kernel.org>
+ *
+ * SPDX-License-Identifier:  BSD-3-Clause
+ */
+
+
+#ifndef SHADOW_INCLUDE_LIB_MEMPCPY_H_
+#define SHADOW_INCLUDE_LIB_MEMPCPY_H_
+
+
+#include <config.h>
+
+#if !defined(HAVE_MEMPCPY)
+
+#include <stddef.h>
+#include <string.h>
+
+
+inline void *mempcpy(void *restrict dst, const void *restrict src, size_t n);
+
+
+inline void *
+mempcpy(void *restrict dst, const void *restrict src, size_t n)
+{
+	return memcpy(dst, src, n) + n;
+}
+
+
+#endif  // !HAVE_MEMPCPY
+#endif  // include guard