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

This change force us to use the top-level CMake's detected 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).

llvm-ranlib runs into problems on AIX 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.

Differential Revision: https://reviews.llvm.org/D142727

Author: daltenty

llvm/cmake/modules/LLVMExternalProjectUtils.cmake

@@ -239,7 +239,7 @@ function(llvm_ExternalProject_Add name source_dir)
     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=${CMAKE_CXX_COMPILER}
@@ -251,6 +251,8 @@ function(llvm_ExternalProject_Add name source_dir)
                       -DCMAKE_OBJDUMP=${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")