Fix linkcheck issues

Most of these are because alloc uses `#[lang_item]` to define methods,
but core documents primitives before those methods are available.

- Fix rustdoc-js-std test

  For some reason this change made CStr not show up in the results for
  `str,u8`. Since it still shows up for str, and since it wasn't a great
  match for that query anyway, I think this is ok to let slide.

- Add test that all primitives can be linked to
- Enable `doc(primitive)` in `core` as well
- Add linkcheck exception specifically for Windows

  Ideally this would be done automatically by the linkchecker by
  replacing `\\` with forward slashes, but this PR is already a ton of
  work ...

- Don't forcibly fail linkchecking if there's a broken intra-doc link on Windows

  Previously, it would exit with a hard error if a missing file had `::`
  in it. This changes it to report a missing file instead, which allows
  adding an exception.

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