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

[Jonas Devlieghere via llvm-commits]

https://github.com/JDevlieghere approved this pull request.

LGTM. The formatting issues are the result of the surrounding code not being compliant. Personally don't think it's worth the churn but even if it were that's something to be fixed in a separate PR. 

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