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

[Hans Wennborg via llvm-commits]

zmodem wrote:

> #208540 is the resolved combination

Thanks! With that (i.e. at 70787a4b5d2824f7cd4cefb8abb718bde073c702) the build works.

It looks like https://github.com/llvm/llvm-project/pull/208421, https://github.com/llvm/llvm-project/pull/208496, and https://github.com/llvm/llvm-project/pull/208540 are still in review (and there's some discussion on the middle one), so I'll revert this for now to unbreak the tree until all the fixes are ready: https://github.com/llvm/llvm-project/pull/208644

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