depends: add missing Darwin objcopy

Our CMake toolchain for a Darwin cross build currently contains:
```bash
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.

Author: fanquake

depends/hosts/darwin.mk

@@ -19,6 +19,7 @@ clangxx_prog=$(shell $(SHELL) $(.SHELLFLAGS) "command -v clang++")
 darwin_AR=$(shell $(SHELL) $(.SHELLFLAGS) "command -v llvm-ar")
 darwin_DSYMUTIL=$(shell $(SHELL) $(.SHELLFLAGS) "command -v dsymutil")
 darwin_NM=$(shell $(SHELL) $(.SHELLFLAGS) "command -v llvm-nm")
+darwin_OBJCOPY=$(shell $(SHELL) $(.SHELLFLAGS) "command -v llvm-objcopy")
 darwin_OBJDUMP=$(shell $(SHELL) $(.SHELLFLAGS) "command -v llvm-objdump")
 darwin_RANLIB=$(shell $(SHELL) $(.SHELLFLAGS) "command -v llvm-ranlib")
 darwin_STRIP=$(shell $(SHELL) $(.SHELLFLAGS) "command -v llvm-strip")