rustdoc: Correctly handle built-in compiler proc-macros as proc-macro and not macro
Part of https://github.com/rust-lang/rust/issues/110111.
There were actually one issue split in two parts:
* Compiler built-in proc-macro were incorrectly considered as macros and not proc-macros.
* Re-exports of compiler built-in proc-macros were considering them as macros.
Both issues can be fixed by looking at the `MacroKind` variant instead of just relying on information extracted later on.
r? ``@fmease``
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