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

[via cfe-commits]

mintsuki wrote:

I would remove the `-kernel` environment, only leaving the `-mlibc` environment which makes sense, similar to `-gnu` and `-musl`.

The rest seems fine to me, but I would change the `defined(__managarm__)` for `defined(__mlibc__)` wherever possible (adding that macro to mlibc if it hasn't been already).

If the notoriety of managarm is an issue for this addition, I would still strongly recommend perhaps removing the managarm portion and leaving `-mlibc` as a new environment. mlibc is used by dozens of OSes at this point, some of which relatively notable.

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