Cleanup `mk/build.mk.sample`

The goal is to make build.mk.sample easier to understand for GHC developers.

There are currently 13 BuildFlavours available to us. It should be clear what the differences between them are, and if those differences are put there on purpose or it they are bugs. There is a certain amount of cargo culting going on in build.mk.sample, with settings just left alone for years because nobody dares touching them (-H64m -fasm) (or nobody cares).

Quiz question: what is the difference between the quick and the devel2 build is.

We should try to keep the changes backward-compatible, because there are blog posts etc out there that refer to the existing BuildFlavours.