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

[Matt Arsenault via cfe-commits]

arsenm wrote:

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

Ideally it would be the normalized triple clang will rewrite this to, so:
```
$ clang --target=aarch64-linux-android --print-target-triple
aarch64-unknown-linux-android
```

Basically we have to build a whole lot of complexity to tolerate the fact that gcc historically allowed spelling the target name in too many different ways. This PR attempted to do handle this by letting clang report the normalize triple, but that requires having an executable clang on the host 

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