[CMake] Use CMAKE_OBJCOPY and CMAKE_STRIP to externalize debug info

[phosek]

Summary

Don't hardcode objcopy and strip names, rather use CMAKE_OBJCOPY and
CMAKE_STRIP variables which allows users to override the tools used
such as using llvm-objcopy and llvm-strip instead of binutils versions.

llvm/trunk/cmake/modules/AddLLVM.cmake

function(llvm_externalize_debuginfo name)
   if(NOT LLVM_EXTERNALIZE_DEBUGINFO)
     return()
   endif()
 
   if(NOT LLVM_EXTERNALIZE_DEBUGINFO_SKIP_STRIP)
     if(APPLE)
       set(strip_command COMMAND xcrun strip -Sxl $<TARGET_FILE:${name}>)
     else()
-      set(strip_command COMMAND strip -gx $<TARGET_FILE:${name}>)
+      set(strip_command COMMAND ${CMAKE_STRIP} -gx $<TARGET_FILE:${name}>)
     endif()
   endif()
 
   if(APPLE)
     if(CMAKE_CXX_FLAGS MATCHES "-flto"
       OR CMAKE_CXX_FLAGS_${uppercase_CMAKE_BUILD_TYPE} MATCHES "-flto")
 
       set(lto_object ${CMAKE_CURRENT_BINARY_DIR}/${CMAKE_CFG_INTDIR}/${name}-lto.o)
       set_property(TARGET ${name} APPEND_STRING PROPERTY
         LINK_FLAGS " -Wl,-object_path_lto,${lto_object}")
     endif()
     add_custom_command(TARGET ${name} POST_BUILD
       COMMAND xcrun dsymutil $<TARGET_FILE:${name}>
       ${strip_command}
       )
   else()
     add_custom_command(TARGET ${name} POST_BUILD
-      COMMAND objcopy --only-keep-debug $<TARGET_FILE:${name}> $<TARGET_FILE:${name}>.debug
+      COMMAND ${CMAKE_OBJCOPY} --only-keep-debug $<TARGET_FILE:${name}> $<TARGET_FILE:${name}>.debug
       ${strip_command} -R .gnu_debuglink
-      COMMAND objcopy --add-gnu-debuglink=$<TARGET_FILE:${name}>.debug $<TARGET_FILE:${name}>
+      COMMAND ${CMAKE_OBJCOPY} --add-gnu-debuglink=$<TARGET_FILE:${name}>.debug $<TARGET_FILE:${name}>
       )
   endif()
 endfunction()
 
 function(llvm_setup_rpath name)
   if(CMAKE_INSTALL_RPATH)
     return()
   endif()