haskell.compiler.ghc902: Switch to LLVM 12
haskell.compiler.ghc902: Switch to LLVM 12

Currently, the build on MacOS ARM64 is broken because LLVM 9 (or more
specifically compiler-rt) is broken (and is marked broken). Both
8.10.7 and 9.2.1 are already set to LLVM 12 so this PR adjusts this to
also use LLVM 12 for GHC 9.0.2 which seems to get things building for me.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs