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

[Juergen Ributzka via llvm-commits]

ributzka wrote:

Since this change also adds MC support, I think this commit should also add the MC build version test for visionOS/xrOS. Alternatively, the MC support and test could be added in a separate commit.

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