[clang] [llvm] runtimes: Pass CMAKE_SYSTEM_NAME based on target triple (PR #203504)

[Matt Arsenault via cfe-commits]

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@@ -376,6 +383,22 @@ function(llvm_ExternalProject_Add name source_dir)
     list(APPEND compiler_args -DCMAKE_CXX_COMPILER_TARGET=${ARG_TARGET_TRIPLE})
     list(APPEND compiler_args -DCMAKE_Fortran_COMPILER_TARGET=${ARG_TARGET_TRIPLE})
     list(APPEND compiler_args -DCMAKE_ASM_COMPILER_TARGET=${ARG_TARGET_TRIPLE})
+
+    # Pass CMAKE_SYSTEM_NAME derived from the target triple so the sub-build
+    # loads the correct platform files instead of the host's.
+    if(NOT "${_cmake_system_name}" STREQUAL "${CMAKE_HOST_SYSTEM_NAME}")
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arsenm wrote:

Maybe this should just be unconditional, I guess it's not worth saving a redundant argument

https://github.com/llvm/llvm-project/pull/203504