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

[Adam Gastineau via llvm-commits]

agg23 wrote:

These changes seem necessary to build (and otherwise seem like they're required at least for consistency): https://github.com/rust-lang/llvm-project/commit/a6cf499f24b5fbe7f1028518f1a972c0176bd6a3

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