depends: add missing Darwin objcopy
Our CMake toolchain for a Darwin cross build currently contains:
set(CMAKE_AR "/usr/bin/llvm-ar")
set(CMAKE_RANLIB "/usr/bin/llvm-ranlib")
set(CMAKE_STRIP "/usr/bin/llvm-strip")
set(CMAKE_OBJCOPY "arm64-apple-darwin-objcopy")
set(CMAKE_OBJDUMP "/usr/bin/llvm-objdump")
objcopy isn't currently used for the Darwin build (only for Linux and splitting the debug symbols), but we shouldn't be producing a toolchain file that refers to nonexistent tools.