title: Enable additional targets with system-provided rust packages, not rustup
body: <p>I wanted to put together a Rust toolchain using the system package manager rather than downloading the rustup installer, and cross-compile for a Raspberry Pi 3</p>
  <p>To that end, in Ubuntu 20.04 I have installed <code>rustc</code> and <code>gcc-arm-linux-gnueabihf</code>.</p>
  <p>However, the <code>cargo build --bins --release --target armv7-unknown-linux-gnueabihf</code> command fails with</p>
  <pre><code>error[E0463]: can't find crate for `core`
    |
    = note: the `armv7-unknown-linux-gnueabihf` target may not be installed
  </code></pre>
  <p>Is there a way to get this working, other than <code>curl https://sh.rustup.rs -sSf | sh &amp;&amp; rustup target add arm-unknown-linux-gnueabihf</code>?</p>