sys: use `process::abort()` instead of `arch::wasm32::unreachable()`
Rationale:
- `abort()` lowers to `wasm32::unreachable()` anyway.
- `abort()` isn't `unsafe`.
- `abort()` matches the comment better.
- `abort()` avoids confusion by future readers (e.g. https://github.com/rust-lang/rust/pull/81527): the naming of wasm's `unreachable` instruction is a bit unfortunate because it is not related to the `unreachable()` intrinsic (intended to trigger UB).
Codegen is likely to be different since `unreachable()` is `inline` while `abort()` is `cold`. Since it doesn't look like we are expecting here to trigger this case, the latter seems better anyway.
76be6bb4de