Use experimental,development and a stable branches

[staffano]

The stable and development branches should not follow any heads in upstream repos, since we need to be able to build reproducable results. The stable is only updated with important updates and should basically follow the second index on gcc's version. The "2" in 8.2.

The development branch follows gcc's stable releases of the latest branch of gcc. Currently 9.1.

The experimental branch follows the heads of the default branches for as many components as possible.