[CMake] Passthrough CMAKE_SYSTEM_NAME to default builtin and runtimes target

[phosek]

Summary

When building the default builtin and runtimes target, set the
CMAKE_SYSTEM_NAME to the current one. This is not necessary on
Linux and Darwin, but it appears to be necessary on Windows,
otherwise CMake fails.

[smeenai]

LGTM

This seems like strange behavior on CMake's part, but the change is fine.

[vvereschaka]

Hello Petr,

these changes break the Windows to ARM Linux cross builders:
http://lab.llvm.org:8011/builders/llvm-clang-win-x-armv7l/builds/4166
http://lab.llvm.org:8011/builders/llvm-clang-win-x-aarch64/builds/4144

We pass a proper CMAKE_SYSTEM_NAME for the target build through BUILTINS_CMAKE_ARGS and RUNTIMES_CMAKE_ARGS variables, such as

set(BUILTINS_CMAKE_ARGS  "-DCMAKE_SYSTEM_NAME=Linux" CACHE STRING "")
set(RUNTIMES_CMAKE_ARGS  "-DCMAKE_SYSTEM_NAME=Linux" CACHE STRING "")

(see clang/cmake/caches/CrossWinToARMLinux.cmake for details)

Your change pass a host system name that is Windows in our case and it is incorrect for the build target.

Is there a way to pass a proper system name in that case?

llvm/runtimes/CMakeLists.txt

llvm_ExternalProject_Add(builtins
                              CMAKE_ARGS -DLLVM_LIBRARY_OUTPUT_INTDIR=${LLVM_LIBRARY_DIR}
                                         -DLLVM_RUNTIME_OUTPUT_INTDIR=${LLVM_TOOLS_BINARY_DIR}
                                         -DLLVM_DEFAULT_TARGET_TRIPLE=${TARGET_TRIPLE}
                                         -DLLVM_ENABLE_PER_TARGET_RUNTIME_DIR=ON
                                         -DCMAKE_C_COMPILER_TARGET=${TARGET_TRIPLE}
                                         -DCMAKE_ASM_COMPILER_TARGET=${TARGET_TRIPLE}
                                         -DCMAKE_C_COMPILER_WORKS=ON
                                         -DCMAKE_ASM_COMPILER_WORKS=ON
+                                        -DCMAKE_SYSTEM_NAME=${CMAKE_SYSTEM_NAME}
                              PASSTHROUGH_PREFIXES COMPILER_RT
                              USE_TOOLCHAIN
                              ${EXTRA_ARGS})
   endfunction()
 
   function(builtin_register_target compiler_rt_path target)
     cmake_parse_arguments(ARG "" "" "DEPENDS" ${ARGN})
 
llvm_ExternalProject_Add(runtimes
                                         -DLLVM_ENABLE_PROJECTS_USED=${LLVM_ENABLE_PROJECTS_USED}
                                         -DLLVM_ENABLE_PER_TARGET_RUNTIME_DIR=ON
                                         -DCMAKE_C_COMPILER_TARGET=${TARGET_TRIPLE}
                                         -DCMAKE_CXX_COMPILER_TARGET=${TARGET_TRIPLE}
                                         -DCMAKE_ASM_COMPILER_TARGET=${TARGET_TRIPLE}
                                         -DCMAKE_C_COMPILER_WORKS=ON
                                         -DCMAKE_CXX_COMPILER_WORKS=ON
                                         -DCMAKE_ASM_COMPILER_WORKS=ON
+                                        -DCMAKE_SYSTEM_NAME=${CMAKE_SYSTEM_NAME}
                              PASSTHROUGH_PREFIXES LLVM_ENABLE_RUNTIMES
                                                   ${ARG_PREFIXES}
                              EXTRA_TARGETS ${extra_targets}
                                            ${test_targets}
                                            ${SUB_COMPONENTS}
                                            ${SUB_CHECK_TARGETS}
                                            ${SUB_INSTALL_TARGETS}
                              USE_TOOLCHAIN