LLVM’s `LLVM_TARGETS_TO_BUILD` build flag defauls to `all`, which contains
`AMDGPU` among others. [1] Changes in llvm [2] switched to explicitly listing
host and target platforms, excluding the AMDGPU target, which is required
for Mesa to build.
[1]: db50b6fe39/CMakeLists.txt (L286-L302)
[2]: https://github.com/NixOS/nixpkgs/pull/52031
ff22d90308