ghc-{8.2.2,8.4.3}: "useLLVM" when target is musl as well

[dtzWill]

Without this build fails complaining unable to find LLVM,
with this option cross-x86_64-musl builds ghc's that at least
can build a simple hello-world "hello.hs".

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Fits CONTRIBUTING.md.

---

[dtzWill]

Building packages (including "hello") seems to run into problems,
but this is a start!

Here's the error when trying to build "hello": https://gist.github.com/09a713cf27a317a9667831a95e4a1483

Oh! ghc843 doesn't have the fixes introduced with #37598 . Let's see how 822 does...

[dtzWill]

With this ghc822 works (again?) with cross-musl (targeting x86_64-unknown-linux-musl or aarch64-multiplatform-musl, at least for the simple "hello" package!

[dtzWill]

Well this looks safe (borg says 0 rebuilds) so going ahead!

LMK if you have any thoughts/comments, thanks!