Fix the OS string encoding for GNU/Hurd

[sthibaul]

Following https://github.com/haskell/cabal/pull/9434/files , and as seen in the various gnu_HOST_OS usages in the ghc source code, it is expected that GNU/Hurd is advertised as "gnu", like the autotools do.

Template Α: This PR modifies behaviour or interface

Include the following checklist in your PR:

• Patches conform to the coding conventions.
• Any changes that could be relevant to users have been recorded in the changelog.
  • Is the change significant? If so, remember to add significance: significant in the changelog file.
• The documentation has been updated, if necessary.
• Manual QA notes have been included.
• Tests have been added. (Ask for help if you don’t know how to write them! Ask for an exemption if tests are too complex for too little coverage!)

[ffaf1]

Mhhhh, I think this modifies behaviour (e.g. creating different folders).

[sthibaul]

Mhhhh, I think this modifies behaviour (e.g. creating different folders).

More precisely, it fixes the behavior. The folders were gnu previously. It's the way OS strings are converted that changed at some point and ended up using hurd instead of gnu, breaking the package build. (i.e. it was never actually built with the changed directory)

[ulysses4ever]

A fix is a change of behaviour. So, it needs a different PR template which, at least, includes a changelog entry.

[sthibaul]

A fix is a change of behaviour. So, it needs a different PR template which, at least, includes a changelog entry.

The broken behaviour could not have been seen by users since ghc would just not compile.

Anyway, I have updated so.

[ffaf1]

Many thanks!