Update the outdated instructions in HACKING.md on how to compile GHC

The instructions in HACKING.md still refer to the old make-based build system. Since they are likely to go stale if unmaintained, I have replaced them with links to the Wiki. I also removed the passage which states that small patches can go via the Github mirror, since the last time a PR was succesfully merged via the Github Mirror seems to be in 2018, i.e. nobody is using this possibility anyways.