haskell.packages.ghc984Binary: init
The 9.8 binaries introduce support for aarch64 musl. They can also be
used to build ghcHEAD, which previously wasn't buildable with a
bindist, so use it there and use the source-built 9.8.4 on platforms
without a bindist for consistency. It fails to build 9.10.x and
9.12.x though, so I've left those as they are.