Rollup merge of #91641 - dtolnay:cchar-if, r=Mark-Simulacrum

Define c_char using cfg_if rather than repeating 40-line cfg

Libstd has a 40-line cfg that defines the targets on which `c_char` is unsigned, and then repeats the same cfg with `not(…)` for the targets on which `c_char` is signed.

This PR replaces it with a `cfg_if!` in which an `else` takes care of the signed case.

I confirmed that `x.py doc library/std` inlines the type alias because c_char_definition is not a publicly accessible path:

![Screenshot from 2021-12-07 13-42-07](https://user-images.githubusercontent.com/1940490/145110596-f1058406-9f32-44ff-9a81-1dfd19b4a24f.png)

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