Rollup merge of #144667 - scottmcm:alignment-is-usize, r=tgross35

`AlignmentEnum` should just be `repr(usize)` now

These used to use specific sizes because they were compiled on all widths.  But now that the types themselves are `#[cfg]`'d, we can save some conversions by having it always be `repr(usize)`.

Author: Zalathar