hadrian: enable building docs for cross targets

Hadrian used to omit the docs target as a part of binary-dist-dir for
cross targets. This commit enables docs to be built as a part of cross
bindists and it works just fine in CI.

(cherry picked from commit a794e733)

hadrian/src/Rules/BinaryDist.hs

@@ -159,7 +159,7 @@ bindistRules = do
 
         let lib_exe_targets = (lib_targets ++ (map (\(_, p) -> p) (bin_targets ++ iserv_targets)))
 
-        let doc_target = if cross then [] else ["docs"]
+        let doc_target = ["docs"]
 
         let other_targets = map (bindistFilesDir -/-) (["configure", "Makefile"] ++ bindistInstallFiles)
         let all_targets = lib_exe_targets ++ doc_target ++ other_targets