[llvm] [llvm] Introduce XROS platform (PR #77707)

[Cyndy Ishida via llvm-commits]

cyndyishida wrote:

> These changes seem necessary to build (and otherwise seem like they're required at least for consistency): [rust-lang at a6cf499](https://github.com/rust-lang/llvm-project/commit/a6cf499f24b5fbe7f1028518f1a972c0176bd6a3)
> 
> Turns out I misunderstood. This PR is missing many changes that are necessary to build LLVM with xrOS support. I'm not sure what milestone these changes represent, but in any case I don't think these are directly actionable.

Correct. I suspect https://github.com/llvm/llvm-project/pull/78373 covers the rest of the changes necessary, at least under the `llvm` subdirectory.  For what it is worth, it seems the fork/branch being used to cherry-pick this PR is behind llvm's TOT main and requires some cherry-picks for this build cleanly. Does rust-lang build against a more stable release branch of the llvm-project? 


https://github.com/llvm/llvm-project/pull/77707