[cmake] Move CMAKE_(C|CXX)_COMPILER variables before CROSS_TOOLCHAIN_FLAGS so they can be overridden when cross compiling.

[hintonda]

Summary

Since CROSS_TOOLCHAN_FLAGS can set CMAKE_(C|CXX)_COMPILER
variables, move the compiler variables up front so they can be
overridden.

This is a followup to https://reviews.llvm.org/D40229 committed in rL319620.

Thanks to Pavel Labath for reporting this issue.

[labath]

Thank you for fixing this on such a short notice.

If this were up to me I'd also move ${build_type_flags} and ${linker_flag} before the CROSS_TOOLCHAIN_FLAGS but that's not my immediate concern.

llvm/trunk/cmake/modules/CrossCompile.cmake

add_custom_command(OUTPUT ${LLVM_${target_name}_BUILD}
     COMMAND ${CMAKE_COMMAND} -E make_directory ${LLVM_${target_name}_BUILD}
     COMMENT "Creating ${LLVM_${target_name}_BUILD}...")
 
   add_custom_target(CREATE_LLVM_${target_name}
                     DEPENDS ${LLVM_${target_name}_BUILD})
 
   add_custom_command(OUTPUT ${LLVM_${target_name}_BUILD}/CMakeCache.txt
     COMMAND ${CMAKE_COMMAND} -G "${CMAKE_GENERATOR}"
-        ${CROSS_TOOLCHAIN_FLAGS_${target_name}} ${CMAKE_SOURCE_DIR}
-        -DLLVM_TARGET_IS_CROSSCOMPILE_HOST=TRUE
         -DCMAKE_C_COMPILER=${CMAKE_C_COMPILER}
         -DCMAKE_CXX_COMPILER=${CMAKE_CXX_COMPILER}
+        ${CROSS_TOOLCHAIN_FLAGS_${target_name}} ${CMAKE_SOURCE_DIR}
+        -DLLVM_TARGET_IS_CROSSCOMPILE_HOST=TRUE
         -DLLVM_TARGETS_TO_BUILD=Native
         ${build_type_flags} ${linker_flag} ${external_clang_dir}
     WORKING_DIRECTORY ${LLVM_${target_name}_BUILD}
     DEPENDS CREATE_LLVM_${target_name}
     COMMENT "Configuring ${target_name} LLVM...")
 
   add_custom_target(CONFIGURE_LLVM_${target_name}
                     DEPENDS ${LLVM_${target_name}_BUILD}/CMakeCache.txt)
 
 endfunction()
 
 function(llvm_create_cross_target target_name sysroot)
   llvm_create_cross_target_internal(${target_name} ${sysroot} ${CMAKE_BUILD_TYPE})
 endfunction()
 
 llvm_create_cross_target_internal(NATIVE "" Release)