Binaries for build->build tools

[Ericson2314]

The cross documentation mentions the [binaries] section of the cross file, along with native : true for building build tools. But how are the tools for those native builds chosen? Just as autoconf has support for various *_FOR_BUILD variables (like CC_FOR_BUILD), I think it would be best to support [binaries_for_build] for that purpose. Maybe [binaries] could be renamed [binaries_for_host] for consistency, too.

CC @eternaleye

[Ericson2314]

NixOS/nixpkgs#39936 (comment) leads me to wonder what CC and friends do in the presence of a cross-file.conf. If those variables do indeed set the native tools, that's a fairly serious inconsistency.

[nirbheek]

Let's discuss this on #3969.

[Ericson2314]

Quite separate from #3969, native files have a [binaries] section, so this is totally done.