Fix the platform string for GNU/Hurd

As commited in Cargo
https://github.com/haskell/cabal/pull/9434
there is confusion between "gnu" and "hurd". This got fixed in Cargo, we
need the converse in Hadrian.

Fixes #24180

hadrian/src/Hadrian/Haskell/Cabal.hs

@@ -75,5 +75,6 @@ cabalOsString :: String -> String
 cabalOsString "mingw32"  = "windows"
 cabalOsString "darwin"   = "osx"
 cabalOsString "solaris2" = "solaris"
+cabalOsString "gnu"      = "hurd"
 cabalOsString other      = other