check_attr: cover multi-segment attributes on specific check arms

PR #128581 introduced an assertion that all builtin attributes are
actually checked via `CheckAttrVisitor` and aren't accidentally usable
on completely unrelated HIR nodes. Unfortunately, the check had
correctness problems.

The match on attribute path segments looked like

```rust,ignore
[sym::should_panic] => /* check is implemented */
match BUILTIN_ATTRIBUTE_MAP.get(name) {
    // checked below
    Some(BuiltinAttribute { type_: AttributeType::CrateLevel, .. }) => {}
    Some(_) => {
        if !name.as_str().starts_with("rustc_") {
            span_bug!(
                attr.span,
                "builtin attribute {name:?} not handled by `CheckAttrVisitor`"
            )
        }
    }
    None => (),
}
```

However, it failed to account for edge cases such as an attribute whose:

1. path segments *starts* with a builtin attribute such as
   `should_panic`
2. which does not start with `rustc_`, and
3. is also an `AttributeType::Normal` attribute upon registration with
   the builtin attribute map

These conditions when all satisfied cause the span bug to be issued for e.g.
`#[should_panic::skip]` because the `[sym::should_panic]` arm is not matched (since it's
`[sym::should_panic, sym::skip]`).

See <https://github.com/rust-lang/rust/issues/128622>.

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