Merge bitcoin/bitcoin#31857: depends: avoid an unset CMAKE_OBJDUMP

2434aea depends: avoid an unset CMAKE_OBJDUMP (fanquake)

Pull request description:

  Similar to #31840, currently our Linux toolchain file contains:
  ```bash
  set(CMAKE_AR "aarch64-linux-gnu-ar")
  set(CMAKE_RANLIB "aarch64-linux-gnu-ranlib")
  set(CMAKE_STRIP "aarch64-linux-gnu-strip")
  set(CMAKE_OBJCOPY "aarch64-linux-gnu-objcopy")
  set(CMAKE_OBJDUMP "")
  ```

  `objdump` is currently only used for the macOS cross build, where it's `llvm-objdump`, but we should be consistent in producing a toolchain file that points to actual tools, rather than leaving variables unset.

ACKs for top commit:
  hebasto:
    ACK 2434aea.
  theuni:
    utACK 2434aea

Tree-SHA512: 65f6b7b9cae79e9c0784c108709139125e52d8f2818afbea5f719bc1b6dc338b870abbdfcb174ae541c0027a7ac07cb56012735b7a37b58b9a6e55a48c0257cf

Author: fanquake

depends/hosts/default.mk

@@ -9,6 +9,7 @@ default_host_RANLIB = $(host_toolchain)ranlib
 default_host_STRIP = $(host_toolchain)strip
 default_host_NM = $(host_toolchain)nm
 default_host_OBJCOPY = $(host_toolchain)objcopy
+default_host_OBJDUMP = $(host_toolchain)objdump
 
 define add_host_tool_func
 ifneq ($(filter $(origin $1),undefined default),)