pkgs/top-level/release.nix: build pkgsLLVM.stdenv as part of trunk
pkgs/top-level/release.nix: build pkgsLLVM.stdenv as part of trunk

Having pkgsLLVM.stdenv built with nixpkgs:trunk will make building
anything in pkgsLLVM decidedly less painful since it will eliminate
the need to build LLVM and clang locally, which shouldn't be as bad
on hydra.

Darwin is disabled for now since it doesn't evaluate correctly there
(infinite recursion problem with the SDK).

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