[AIX][CMake] Use top-level tools in llvm_ExternalProject_Add

[daltenty]

Summary

This change force us to use the top-level CMake's detect tools. We need to do this as a temporary workaround as when using CMake versions >= 3.22 we'll pickup the built llvm-ranlib by default if it's in the path (which it is when doing a sub build via llvm_ExternalProject_Add for the runtimes), but llvm-ran runs into problems due to missing 64-bit functionality to be added by https://reviews.llvm.org/D142479 and https://reviews.llvm.org/D142660. Once those patches land, this can be reverted.

[DiggerLin]

when using CMake versions >= 3.22 we'll pickup the built llvm-ranlib by default if it's in the path (which it is when doing a sub build via llvm_ExternalProject_Add for the runtimes),

The value "USE_TOOLCHAIN" is empty when calling the function llvm_ExternalProject_Add() in the runtimes/CMakeLists.txt.
and also I do not find the "llvm-ranlib" are set to the TOOLCHAIN or set -DCMAKE_RANLIB to any value in function llvm_ExternalProject_Add() for AIX OS (before your patch.) . you means that if there is no define of -DCMAKE_RANLIB= , the CMake (versions >= 3.22) will use llvm-ranlib by default?

[daltenty]

In D142727#4091417, @DiggerLin wrote:

you means that if there is no define of -DCMAKE_RANLIB= , the CMake (versions >= 3.22) will use llvm-ranlib by default?

Yep, if it can find it in the path and the toolchain is clang-based it seems too. I haven't narrowed down the exact CMake change that did this, but I suspect it might be related to LTO / bitcode archive support work.

[DiggerLin]

LGTM.

llvm/cmake/modules/LLVMExternalProjectUtils.cmake

function(llvm_ExternalProject_Add name source_dir)
   if(ARG_EXCLUDE_FROM_ALL)
     set(exclude EXCLUDE_FROM_ALL 1)
   endif()
 
   if(CMAKE_SYSROOT)
     set(sysroot_arg -DCMAKE_SYSROOT=${CMAKE_SYSROOT})
   endif()
 
-  if(CMAKE_CROSSCOMPILING)
+  if(CMAKE_CROSSCOMPILING OR _cmake_system_name STREQUAL AIX)
     set(compiler_args -DCMAKE_ASM_COMPILER=${CMAKE_ASM_COMPILER}
                       -DCMAKE_C_COMPILER=${CMAKE_C_COMPILER}
                       -DCMAKE_CXX_COMPILER=${CMAKE_CXX_COMPILER}
                       -DCMAKE_LINKER=${CMAKE_LINKER}
                       -DCMAKE_AR=${CMAKE_AR}
                       -DCMAKE_RANLIB=${CMAKE_RANLIB}
                       -DCMAKE_NM=${CMAKE_NM}
                       -DCMAKE_OBJCOPY=${CMAKE_OBJCOPY}
                       -DCMAKE_OBJDUMP=${CMAKE_OBJDUMP}
                       -DCMAKE_STRIP=${CMAKE_STRIP}
                       -DCMAKE_READELF=${CMAKE_READELF})
+  endif()
+  if(CMAKE_CROSSCOMPILING)
     set(llvm_config_path ${LLVM_CONFIG_PATH})
 
     if(CMAKE_CXX_COMPILER_ID MATCHES "Clang")
       string(REGEX MATCH "^[0-9]+" CLANG_VERSION_MAJOR
              ${PACKAGE_VERSION})
       set(resource_dir "${LLVM_LIBRARY_DIR}/clang/${CLANG_VERSION_MAJOR}")
       set(flag_types ASM C CXX MODULE_LINKER SHARED_LINKER EXE_LINKER)
       foreach(type ${flag_types})