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

[Alexander van der Grinten via llvm-commits]

avdgrinten wrote:

Regarding the acceptance bar, I'd like to stress the following point: it is true that Managarm is not a mainstream OS (but neither are some of the other OSes that have existing LLVM and Clang targets); however, LLVM is an infrastructure project that is used by a lot of downstreams (e.g., out of tree compilers), and not being supported by LLVM seriously impacts our ability to integrate with downstream projects.

In fact, maintaining our LLVM patches has been quite easy ([and we've been doing that out-of-tree since 2019](https://github.com/managarm/bootstrap-managarm/commit/565104262b7aa96995c80c86b6857ae872da6efa)), but maintaining patches for downstream compilers tends to be much more involved. This is especially true for Rust, where the mainstream tooling is not built in a way that allows us to easily apply out-of-tree patches to packages managed by `cargo` etc. One of the main reasons why we want to get into upstream LLVM now is that we also have a Rust port that we want to upstream, and upstream Rust wants us to submit the LLVM target first.

We are committed to maintain our LLVM port indefinitely, and we've been doing it out-of-tree already since LLVM 6.

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