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

[Matt Arsenault via cfe-commits]

arsenm wrote:

> We're hitting build errors from the runtimes build after this change. Apologies for not having a tighter repro, but this is still significantly smaller than the full thing:
> 

Is it better with #208421?



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