Update the references in boot to mention GitLab instead of ghc.haskell.org

ghc-8.6.4/boot contains:

• 57: * https://ghc.haskell.org/trac/ghc/wiki/Newcomers or
• 58: * https://ghc.haskell.org/trac/ghc/wiki/Building/GettingTheSources#CloningfromGitHub
• 190: http://ghc.haskell.org/trac/ghc/wiki/Building/Using#Buildconfiguration

and boot prints the latter line. Visiting it states, the Wiki/information is moved to Gitlab.

• Update the references in boot to mention GitLab instead of ghc.haskell.org.