Add cmake to source release tarballs

I've split the git archive generation into three steps:

1. generate pure tarball
2. append top-level cmake directory to all tarballs
3. compress the archive

This was inspired by D118252 and can be considered an alternative
approach for all projects to have access to the shared cmake
directory when building in standalone mode.

When generating source tarballs on my local laptop it takes 9 minutes and 45 seconds WITH this patch applied. When this patch is not applied, it takes 9minutes and 38 seconds. That means, this patch introduces a slowdown of 7 seconds, which seems fair.

Reviewed By: tstellar

Differential Revision: https://reviews.llvm.org/D118481