optimize mempcpy to minimize need for data saved across the call

src/string/mempcpy.c

@@ -2,6 +2,5 @@
 
 void *mempcpy(void *dest, const void *src, size_t n)
 {
-	memcpy(dest, src, n);
-	return (char *)dest + n;
+	return (char *)memcpy(dest, src, n) + n;
 }