Add CLANG_DEFAULT_OBJCOPY to allow Clang to use llvm-objcopy for dwarf fission

llvm-objcopy is getting to where it can be used in non-trivial ways
(such as for dwarf fission in clang). It now supports dwarf fission but
this feature hasn't been thoroughly tested yet. This change allows
people to optionally build clang to use llvm-objcopy rather than GNU
objcopy. By default GNU objcopy is still used so nothing should change.

Differential Revision: https://reviews.llvm.org/D39029

llvm-svn: 317960