binutils: Respect the targetPlatform

Use `buildPackages.binutils` to get build = host != target binutils,
i.e. the old `binutilsCross`, and use
`buildPackages.buildPackages.binutils` to get build = host = target
binutils, i.e. the old `binutils`.

`buildPackages` chains like this are supposed to remove the need for
all such `*Cross` derivations. We start with binutils because it's
comparatively easy.

No hashes of cross-tests should be changed