[CMake] Pass through CMAKE_READELF to subbuilds

[phosek]

Summary

This matches handling of other CMake variables.

clang/CMakeLists.txt

if(BOOTSTRAP_CMAKE_SYSTEM_NAME STREQUAL "Linux")
       if(NOT BOOTSTRAP_LLVM_ENABLE_LTO)
         add_dependencies(clang-bootstrap-deps llvm-ar llvm-ranlib)
         set(${CLANG_STAGE}_AR -DCMAKE_AR=${LLVM_RUNTIME_OUTPUT_INTDIR}/llvm-ar)
         set(${CLANG_STAGE}_RANLIB -DCMAKE_RANLIB=${LLVM_RUNTIME_OUTPUT_INTDIR}/llvm-ranlib)
       endif()
       add_dependencies(clang-bootstrap-deps llvm-objcopy llvm-strip)
       set(${CLANG_STAGE}_OBJCOPY -DCMAKE_OBJCOPY=${LLVM_RUNTIME_OUTPUT_INTDIR}/llvm-objcopy)
       set(${CLANG_STAGE}_STRIP -DCMAKE_STRIP=${LLVM_RUNTIME_OUTPUT_INTDIR}/llvm-strip)
+      set(${CLANG_STAGE}_READELF -DCMAKE_READELF=${LLVM_RUNTIME_OUTPUT_INTDIR}/llvm-readelf)
     endif()
   endif()
 
   if(BOOTSTRAP_LLVM_BUILD_INSTRUMENTED)
     add_dependencies(clang-bootstrap-deps llvm-profdata)
     set(PGO_OPT -DLLVM_PROFDATA=${LLVM_RUNTIME_OUTPUT_INTDIR}/llvm-profdata)
   endif()
 

compiler-rt/cmake/Modules/AddCompilerRT.cmake

set(PASSTHROUGH_VARIABLES
     CMAKE_MAKE_PROGRAM
     CMAKE_LINKER
     CMAKE_AR
     CMAKE_RANLIB
     CMAKE_NM
     CMAKE_OBJCOPY
     CMAKE_OBJDUMP
     CMAKE_STRIP
+    CMAKE_READELF
     CMAKE_SYSROOT
     LIBCXX_HAS_MUSL_LIBC
     PYTHON_EXECUTABLE
     Python3_EXECUTABLE
     Python2_EXECUTABLE
     CMAKE_SYSTEM_NAME)
   foreach(variable ${PASSTHROUGH_VARIABLES})
     get_property(is_value_set CACHE ${variable} PROPERTY VALUE SET)

llvm/cmake/modules/LLVMExternalProjectUtils.cmake

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_STRIP=${CMAKE_STRIP}
+                      -DCMAKE_READELF=${CMAKE_READELF})
     set(llvm_config_path ${LLVM_CONFIG_PATH})
 
     if(CMAKE_CXX_COMPILER_ID MATCHES "Clang")
       string(REGEX MATCH "[0-9]+\\.[0-9]+(\\.[0-9]+)?" CLANG_VERSION
              ${PACKAGE_VERSION})
       set(resource_dir "${LLVM_LIBRARY_DIR}/clang/${CLANG_VERSION}")
       set(flag_types ASM C CXX MODULE_LINKER SHARED_LINKER EXE_LINKER)
       foreach(type ${flag_types})