symbols/us: Add `periodcentered`, `dead_stroke` to us-intl variants

dead_stroke is required to type four letters (eight characters) of Latin Extended-A. Additionally, periodcentered is a standard alternative multiplication symbol within the context of algebra in the United States. Both of which are put on the 3rd and 4th levels of x with the nomenclature "multiply" and "cross-out". However, for variants whose x position is already occupied, namely us-altgr-intl and capalx-intl, they are instead given the nearest unused key position.