The tutorial "Configuring C++ toolchains" uses deprecated mechanisms

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### Description of the problem / feature request:

> The tutorial "Configuring C++ toolchains" uses deprecated mechanisms

### Bugs: what's the simplest, easiest way to reproduce this bug? Please provide a minimal example if possible.

> Follow the tutorial at https://docs.bazel.build/versions/master/tutorial/cc-toolchain-config.html
> Get the following error message:
```
in cc_toolchain rule //asmjs_toolchain: attributes 'cpu' and 'compiler' have been deprecated, please remove them. See https://github.com/bazelbuild/bazel/issues/7075 for details.
```

### What operating system are you running Bazel on?

> Gentoo Linux

### What's the output of `bazel info release`?

> release 0.28.1- (@non-git)

### If `bazel info release` returns "development version" or "(@non-git)", tell us how you built Bazel.

> Built with portage (this shouldn't matter though)

### Have you found anything relevant by searching the web?

> Yeah, the error message itself points to https://github.com/bazelbuild/bazel/issues/7075

Start with the linked “Configuring C++ toolchains” tutorial and compare its cc_toolchain configuration with the deprecation guidance in issue 7075. Reproduce the example with Bazel 0.28.1 or the documented setup, then verify that following the tutorial no longer reports deprecated cpu and compiler attributes.

Written by the indexing model from the issue text.