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

[Matt Arsenault via cfe-commits]

arsenm wrote:

> With a local modification like this on top, my build succeeds again:
> 
> ```diff
> diff --git a/llvm/cmake/modules/LLVMConfig.cmake.in b/llvm/cmake/modules/LLVMConfig.cmake.in
> index 6ef0cef7d029..8eaf659d82c0 100644
> --- a/llvm/cmake/modules/LLVMConfig.cmake.in
> +++ b/llvm/cmake/modules/LLVMConfig.cmake.in
> @@ -147,7 +147,7 @@ set(LLVM_ENABLE_SHARED_LIBS @BUILD_SHARED_LIBS@)
>  set(LLVM_DEFAULT_EXTERNAL_LIT "@LLVM_CONFIG_DEFAULT_EXTERNAL_LIT@")
>  set(LLVM_LIT_ARGS "@LLVM_LIT_ARGS@")
>  
> -if(NOT TARGET LLVMSupport AND NOT CMAKE_CROSSCOMPILING)
> +if(NOT TARGET LLVMSupport)
>    @LLVM_CONFIG_INCLUDE_EXPORTS@
>    @llvm_config_include_buildtree_only_exports@
>  endif()
> ```

Hmm. With this I still get

```
CMake Warning (dev) at /Users/matt/src/llvm-project/build_verify/lib/cmake/llvm/LLVMExports.cmake:1472 (add_library):
  ADD_LIBRARY called with SHARED option but the target platform does not
  support dynamic linking.  Building a STATIC library instead.  This may lead
  to problems.
Call Stack (most recent call first):
  /Users/matt/src/llvm-project/build_verify/lib/cmake/llvm/LLVMConfig.cmake:372 (include)
  CMakeLists.txt:79 (find_package)
This warning is for project developers.  Use -Wno-dev to suppress it.

```

which was the point of this part. I'll submit this as a PR as the quick fix and a follow up with a revised strategy 



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