[clang] [llvm] Reapply "runtimes: Pass CMAKE_SYSTEM_NAME based on target triple" (#205133) (PR #205522)

[Matt Arsenault via cfe-commits]

arsenm wrote:

> I'm afraid not. I cherry-picked both #208420 and #208421 on top of this one, and got the same error.

https://github.com/llvm/llvm-project/pull/208496

https://github.com/llvm/llvm-project/pull/205522