[CMake] Use CMAKE_OBJCOPY and CMAKE_STRIP to externalize debug info

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.

Differential Revision: https://reviews.llvm.org/D46611

llvm-svn: 331827