Changes to UserSettings.hs

[Matthew Pickering]

Hi all,

Last night my patch got merged which changed some aspects of the
hadrian settings file.

If you have a custom settings file (ie hadrian/UserSettings.hs), you
need to remove reference to the `stage1Only` variable and
add a new variable called `finalStage` which should default to `Stage2`.

https://gitlab.haskell.org/ghc/ghc/commit/6486c6e49c53e75f37ed732b38c5be7ae64785e8#b5b7b63ab5532ca7281d44e164d77f0d56c1fddc

Cheers,

Matt