Request native clang only when cross-compiling (#464)

* Request native clang only when cross-compiling

LLVM_USE_HOST_TOOLS may be set if LLVM is configured with
LLVM_OPTIMIZED_TABLEGEN, which does not necessarily indicate
cross-compilation or that clang will only execute on the target.

By checking that CMAKE_CROSSCOMPILING is set, we ensure that we only
build/use clang again if necessary for host execution.

* fixup: CMAKE_CROSSCOMPILING implies LLVM_USE_HOST_TOOLS

Co-authored-by: Wenju He <[email protected]>

* fixup: also use CMAKE_CROSSCOMPILING in top-level CMakeLists.txt

---------

Co-authored-by: Wenju He <[email protected]>

Author: tcreech-intel

CMakeLists.txt

@@ -35,7 +35,7 @@ set(CMAKE_MODULE_PATH
 
 include(CMakeFunctions)
 
-if(LLVM_USE_HOST_TOOLS AND OPENCL_CLANG_BUILD_EXTERNAL)
+if(CMAKE_CROSSCOMPILING AND OPENCL_CLANG_BUILD_EXTERNAL)
   include(CrossCompile)
   llvm_create_cross_target(${PROJECT_NAME} NATIVE "" Release)
 endif()

cl_headers/CMakeLists.txt

@@ -4,7 +4,7 @@ if(USE_PREBUILT_LLVM)
 else()
     set(CLANG_COMMAND $<TARGET_FILE:clang>)
 endif()
-if(LLVM_USE_HOST_TOOLS AND NOT OPENCL_CLANG_BUILD_EXTERNAL)
+if(CMAKE_CROSSCOMPILING AND NOT OPENCL_CLANG_BUILD_EXTERNAL)
   build_native_tool(clang CLANG_COMMAND)
 endif()
 set(LINUX_RESOURCE_LINKER_COMMAND linux_resource_linker)