[clang] [llvm] [llvm] Add triples for managarm (PR #87845)

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no92 wrote:

Rebased and updated the PR to include the feedback. We've decided to drop the `kernel` environment for now, and will maintain it out-of-tree for the time being. We might come up with a better solution for it in the future, also see @ArsenArsen's comment above for details why we use it currently.

Other than that, the implementation was adapted to work along the same lines as Linux and Hurd - in particular, we don't add triplets to the arrays where we ought not to. Tests were added, and we currently use these patches for working on managarm successfully.

AFAICT the CI failure seems unrelated to the PR, but I have a hard time telling.

https://github.com/llvm/llvm-project/pull/87845