Add a Gitlab CI script that runs Circle CI validation jobs

And put it to use for running i386 and x86_64 linux validate,
x86_64 darwin validate and building with hadrian. For all the validate
jobs, we save the artifacts (bindist + test results for now) in Circle CI
and subsequently grab them in Gitlab. The nice side effect is that merge
requests now show the change in test results from the patches and let you
download or browse the artifacts.