Linux platforms use pl-gcc by default

pl-gcc is required by default in runtime components for both puppet-runtime and puppet-agent:

• https://github.com/puppetlabs/puppet-agent/blob/master/configs/components/runtime.rb#L28
• https://github.com/puppetlabs/puppet-runtime/blob/master/configs/components/runtime-agent.rb#L24 

This makes it too easy to rely on pl-gcc in new platform additions; The default behavior should be to use system gcc and fall back to pl-gcc for platforms where the version is inadequate.