Pare down cross-building configuration

[mudge]

In an attempt to make the precompilation of gems easier to maintain, try
to make it as concise as possible.

Specifically:

• Switch the downloading of Abseil and RE2's archives to Rake file tasks
  as they will automatically be required by the default gem task as long
  as they are added to the gem specification.
• Switch from invoking Docker directly in CI to using the
  gem:$platform tasks we define in the Rakefile. These will
  automatically use the correct version of rake-compiler-docker's Docker
  images for us and use rake-compiler's native:$platform tasks and the
  gem package task to build the correct gem.
• Remove the test-gem-build and build-gems tasks in favour of explicitly
  using the appropriate Rake tasks (rake gem:$platform for precompiled
  gems and rake gem for the C Ruby gem).
• We keep the test-gem-install script as it is easier to pass to Docker
  and the FreeBSD VM as a single entrypoint though it now only installs
  the gem and its required dependencies (reusing the Bundler cache
  created by setup-ruby where possible) before running the test suite in
  the gem install directory.