[clang] [llvm] [Flang] Search flang_rt in clang_rt path (PR #151954)

[Michael Kruse via llvm-commits]

Meinersbur wrote:

@PHHargrove Thanks for the details. There is something weird about your setup.
1. `libflang_rt.runtime.a` in `arm64-apple-darwin23.6.0/`. As long as CMake detects [`APPLE`](https://cmake.org/cmake/help/latest/variable/APPLE.html), the install directory will be [`CMAKE_SYSTEM_NAME`](https://cmake.org/cmake/help/latest/variable/CMAKE_SYSTEM_NAME.html). I have no explanation for this.
2. The native target is `arm64-apple-darwin23.6.0`, but you are compiling for `arm64-apple-macosx14.0.0`, probably because you compiled LLVM with an `LLVM_DEFAULT_TARGET_TRIPLE` option, or you updated your OS since you first configured LLVM. This has elements of cross-compilation which is still unsupported for Flang. Consider deleting your build directory a running cmake from scratch.

https://github.com/llvm/llvm-project/pull/151954