Explore toolchain build reproducibility

[theopolis]

We would like the scripts to build a reproducible toolchain (if possible). This allows us to provide integrity for the assets and releases.

[Smjert]

One of the steps I think we should do is to release the stage0 GCC we build, so that we can always reuse that to then build the LLVM toolchain.
This also has the nice side effect that it speeds up the build, since we don't always have to start from scratch.