Merge bitcoin/bitcoin#31840: depends: add missing Darwin objcopy

3edaf0b depends: add missing Darwin objcopy (fanquake)

Pull request description:

  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.

ACKs for top commit:
  laanwj:
    Code review ACK 3edaf0b
  theuni:
    utACK 3edaf0b

Tree-SHA512: b74deb9f3f053c37d03505e698419d4a14131131f12a042dab698a81f2ad76b71fd55c1d1afd5f5822cc50fdaad5acdab15a8b20626c56f705179add1165449f

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")