These binary packages are available for a fixed set of platforms,
all of which support the native code generator. Therefore, the
`llvmPackages` argument was never used. We leave an assertion around,
just in case.
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.
