doc: Update cross compilation chapter for stdenv.{build,host,target}Platform

    </para>

    <para>

      In Nixpkgs, these three platforms are defined as attribute sets under the names <literal>buildPlatform</literal>, <literal>hostPlatform</literal>, and <literal>targetPlatform</literal>.

      All three are always defined at the top level, so one can get at them just like a dependency in a function that is imported with <literal>callPackage</literal>:

      <programlisting>{ stdenv, buildPlatform, hostPlatform, fooDep, barDep, .. }: ...</programlisting>

      All three are always defined as attributes in the standard environment, and at the top level. That means one can get at them just like a dependency in a function that is imported with <literal>callPackage</literal>:

      <programlisting>{ stdenv, buildPlatform, hostPlatform, fooDep, barDep, .. }: ...buildPlatform...</programlisting>, or just off <varname>stdenv</varname>:

      <programlisting>{ stdenv, fooDep, barDep, .. }: ...stdenv.buildPlatform...</programlisting>.

    </para>

    <variablelist>

      <varlistentry>