[CMake] Pass llvm-readelf to CMake external builds

[phosek]

Summary

This matches other LLVM binary tools.

llvm/cmake/modules/LLVMExternalProjectUtils.cmake

if(NOT ARG_TOOLCHAIN_TOOLS)
     if(NOT _cmake_system_name STREQUAL AIX)
       list(APPEND ARG_TOOLCHAIN_TOOLS lld llvm-ar llvm-ranlib llvm-nm llvm-objdump)
       if(_cmake_system_name STREQUAL Darwin)
         list(APPEND ARG_TOOLCHAIN_TOOLS llvm-libtool-darwin llvm-lipo)
       elseif(is_msvc_target)
         list(APPEND ARG_TOOLCHAIN_TOOLS llvm-lib)
       else()
         # TODO: These tools don't fully support Mach-O format yet.
-        list(APPEND ARG_TOOLCHAIN_TOOLS llvm-objcopy llvm-strip)
+        list(APPEND ARG_TOOLCHAIN_TOOLS llvm-objcopy llvm-strip llvm-readelf)
       endif()
     endif()
   endif()
   foreach(tool ${ARG_TOOLCHAIN_TOOLS})
     if(TARGET ${tool})
       list(APPEND TOOLCHAIN_TOOLS ${tool})
 
       # $<TARGET_FILE:tgt> only works on add_executable or add_library targets
if(llvm-objdump IN_LIST TOOLCHAIN_TOOLS)
       list(APPEND compiler_args -DCMAKE_OBJDUMP=${LLVM_RUNTIME_OUTPUT_INTDIR}/llvm-objdump${CMAKE_EXECUTABLE_SUFFIX})
     endif()
     if(llvm-objcopy IN_LIST TOOLCHAIN_TOOLS)
       list(APPEND compiler_args -DCMAKE_OBJCOPY=${LLVM_RUNTIME_OUTPUT_INTDIR}/llvm-objcopy${CMAKE_EXECUTABLE_SUFFIX})
     endif()
     if(llvm-strip IN_LIST TOOLCHAIN_TOOLS AND NOT ARG_STRIP_TOOL)
       list(APPEND compiler_args -DCMAKE_STRIP=${LLVM_RUNTIME_OUTPUT_INTDIR}/llvm-strip${CMAKE_EXECUTABLE_SUFFIX})
     endif()
+    if(llvm-readelf IN_LIST TOOLCHAIN_TOOLS)
+      list(APPEND compiler_args -DCMAKE_READELF=${LLVM_RUNTIME_OUTPUT_INTDIR}/llvm-readelf${CMAKE_EXECUTABLE_SUFFIX})
+    endif()
     list(APPEND ARG_DEPENDS ${TOOLCHAIN_TOOLS})
   endif()
 
   if(ARG_STRIP_TOOL)
     list(APPEND compiler_args -DCMAKE_STRIP=${ARG_STRIP_TOOL})
   endif()
 
   add_custom_command(