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

[Nico Weber via cfe-commits]

nico wrote:

> > -DRUNTIMES_aarch64-linux-android21_LLVM_ENABLE_RUNTIMES
> 
> It's unfortunate this is using an unnormalized triple

To confirm, are you saying that we're holding it wrong? What should we be passing here?

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