title: Bazel: add compile flags to default C++ toolchain
body: <p>I would like to add some compiler and linker flags to the default C++ toolchain, so that all the targets I build (local or imported) share them.</p>
  
  <p>I know that can define my own toolchain, but I don't want to do that as it's very complicated and easy to get wrong.</p>
  
  <p>Ideally I would like something like this:</p>
  
  <pre><code>cc_toolchain = cc_default_toolchain()
  cc_toolchain.copts = [...]
  cc_toolchain.linkopts = [...]
  </code></pre>
  
  <p>Also, I don't want to set global flags in the <code>.bashrc</code> file, as it's hard to configure per platform and it's not easy to share among different repositories.</p>
  
  <p>Thanks!</p>