[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@331827 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed