koka: take C compiler from targetPackages

[sternenseemann]

buildPackages.stdenv.cc.cc is a C compiler that runs on the build
platform and produces binaries for the host platform. This is not
what we want. Also pkgsHostTarget.stdenv.cc is not the compiler we
want as stdenv always runs on the previous stage so to say (the stdenv
is used to build the package set, in the case of cross compiling
this is not done natively). Thus stdenv.targetPackages.stdenv.cc is what
we want (note that I haven't really spent any thought on cross compiling
a koka cross compiler, but it'll be a while until someone needs to do
this).

Testing would be appreciated.

Motivation for this change

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• 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 nixpkgs-review --run "nixpkgs-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)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.