Now that zig1.wasm is updated, apply the matching standard library
changes.
The main one is in `std.builtin.Type`, where `alignment` fields now have
type `?usize` rather than `comptime_int`.
Additionally, we need to add explicit backing integers to some packed
unions, because (due to https://github.com/ziglang/zig/issues/24714)
they need explicit backing integers to be used in `extern` contexts.
This change could not happen before now, because prior to this branch,
packed unions did not allow explicit backing integer types (that is,
this branch implemented https://github.com/ziglang/zig/issues/25350).
Matthew Lugg