systemd: fix evaluation in pkgsCross.ghcjs.buildPackages

GHC's js backend depends on systemd via emscripten via closure compiler
via jdk via cups. Before it fails to evaluate, though, since
llvmPackages looks into `targetPackages.stdenv.cc` to determine which
C++ library to use (something that should be rectified in the future).
[Unfortunately], for `pkgsCross.ghcjs`, `stdenv.cc` throws which blows
up evaluating `pkgsCross.buildPackages.llvmPackages.clang`.

This is in principle unnecessary. We want to build
`pkgsCross.ghcjs.buildPackages.haskell.compiler.native-bignum.ghcHEAD`
which depends on `pkgsCross.ghcjs.buildPackages.systemd` which needs
clang and friends only in `nativeBuildInputs`, so
`pkgsCross.ghcjs.buildPackages.buildPackages.llvmPackages.clang`.
Unfortunately, due to the nature of splicing, we first evaluate the
“adjacent” derivation before we can access the spliced derivation we are
actually interested in. If the former
fails (`pkgsCross.ghcjs.buildPackages.llvmPackages.clang`), we can't do
the latter.

The solution is to just not rely on splicing in this case and take
`buildPackages.llvmPackages.clang` directly (relative to
`buildPackages.systemd` in this case!) which avoids the whole problem.

[Unfortunately]: https://github.com/NixOS/nixpkgs/commit/c739c420db5b9d56c335414be1696c57f2dbbb6a#diff-3209527bd27cbc775f579b1e295b0264c850859c7245d526965cec456b8c70a4R61

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