Use gnuconfig package for config file replacement (#26035)

* Use gnuconfig package for config file replacement

Currently the autotools build system tries to pick up config.sub and
config.guess files from the system (in /usr/share) on arm and power.
This is introduces an implicit system dependency which we can avoid by
distributing config.guess and config.sub files in a separate package,
such as the new `gnuconfig` package which is very lightweight/text only
(unlike automake where we previously pulled these files from as a
backup). This PR adds `gnuconfig` as an unconditional build dependency
for arm and power archs.

In case the user needs a system version of config.sub and config.guess,
they are free to mark `gnuconfig` as an external package with the prefix
pointing to the directory containing the config files:

```yaml
    gnuconfig:
      externals:
      - spec: gnuconfig@master
        prefix: /tmp/tmp.ooBlkyAKdw/lol
      buildable: false
```

Apart from that, this PR gives some better instructions for users when
replacing config files goes wrong.

* Mock needs this package too now, because autotools adds a depends_on

* Add documentation

* Make patch_config_files a prop, fix the docs, add integrations tests

* Make macOS happy

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