[llvm] Runtimes: specify whether to leverage a target triple for default builds (PR #130170)

[Eric Miotto via llvm-commits]

edymtt wrote:

This is not needed -- as per [discussion in forums](https://discourse.llvm.org/t/rfc-amending-runtimes-build-system-to-be-able-to-build-all-apple-platforms-not-only-macos/85019/4), the correct approach here is to leverage the mechanism introduced with https://reviews.llvm.org/D86313, that is not relying on the default targets for builtins and runtimes but passing the following to the CMake invocation (or set them in the cache used)

```
...
-DLLVM_BUILTIN_TARGETS=arm64-apple-darwin \
-DLLVM_RUNTIME_TARGETS=arm64-apple-darwin \
-DRUNTIMES_BUILD_ALLOW_DARWIN=ON \
...
```

https://github.com/llvm/llvm-project/pull/130170