Fix invalid primitive's methods link generation in core and alloc crate

[GuillaumeGomez]

There are some broken links when generating primitives' methods using intra-doc links from the core and alloc crate. The list is here. It was introduced in #87073. At the time we merged because the change was really important and far outweighed this problem. But it would be nice to finish it. :)

[jyn514]

@GuillaumeGomez this is not a problem we can fix in rustdoc. It's a special case of #74481.

[GuillaumeGomez]

I had in mind something like what you did #87073: provide a link through a markdown file depending whether we were in core/alloc or in std. If we are in a downstream crate, then we provide a "hard" link to the method in std. Having a more generic way to do so would be nice though.

[jyn514]

@GuillaumeGomez that's what broke it in the first place. Relative links can never work for both the original item and the re-export.

[jyn514]

I'm going to close this as a duplicate of #32130, I don't think we need multiple issues tracking the linkcheck exceptions.