Move cGHC_UNLIT_PGM to be "unlit command" in settings

The bulk of the work was done in #712, making settings be make/Hadrian
controlled. This commit then just moves the unlit command rules in
make/Hadrian from the `Config.hs` generator to the `settings` generator
in each build system.

I think this is a good change because the crucial benefit is *settings*
don't affect the build: ghc gets one baby step closer to being a regular
cabal executable, and make/Hadrian just maintains settings as part of
bootstrapping.