VS: Teach VS 2019 generator to select host tools matching host arch

This generator is new so we can introduce the long-desired behavior
of selecting ``host=x64`` tools by default on x64 hosts.
11 jobs for <span class="ref-name">vs-host-arch</span>
Status Job ID Name Coverage
  External
passed ghostflow-check-master

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager