ghcHEAD: Fix Windows cross-compilation with lld.

By specifying pkgs.libffi here instead of letting callPackage handle
it, we confuse the splicing logic and put the host libffi into
NIX_LDFLAGS_FOR_TARGET. This previously hasn't been a problem, as we
also pass an explicit configure flag pointing to the target libffi,
and so the only side-effect is a senseless -rpath flag.

ld.lld (rightly) does not recognize the -rpath flag when targeting
Windows, however, so this causes build failures.

pkgs/top-level/haskell-packages.nix

@@ -145,7 +145,6 @@ in {
       inherit (buildPackages.darwin) xattr autoSignDarwinBinariesHook;
       buildTargetLlvmPackages = pkgsBuildTarget.llvmPackages_12;
       llvmPackages = pkgs.llvmPackages_12;
-      libffi = pkgs.libffi;
     };
 
     ghcjs = compiler.ghcjs810;