Example with `clangUseLLVM` which is the default when using `useLLVM`
```nix
config.replaceCrossStdenv = { buildPackages, baseStdenv }:
if baseStdenv.targetPlatform.useLLVM or false
then (buildPackages.stdenvAdapters.overrideCC baseStdenv buildPackages.llvmPackages_16.clangUseLLVM)
else baseStdenv;
```
The conditional necessary, otherwise the other sets(such as `pkgsCross.aarch64-multiplatform.llvmPackages`)
without `useLLVM` will use the stdenv without the necessary conditions to avoid infinite
recursion because of [targetLlvmLibraries](644b234e1c/pkgs/development/compilers/llvm/16/default.nix (L208))
usage.
[`replaceStdenv` is not used when cross-compiling](d77bda728d/pkgs/stdenv/cross/default.nix (L12-L13))
`replaceStdenv` uses an additional stage to replace the stdenv to avoid
infinite recursion and other issues but that should not be necessary for cross.
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