depends: set CMAKE_*_COMPILER_TARGET in toolchain

According to the CMake docs, this is the correct way to setup a
toolchain file for cross-compilation using Clang. See
https://cmake.org/cmake/help/latest/manual/cmake-toolchains.7.html#cross-compiling-using-clang

Internally it looks like CMake will only take this variable into account
if it detects the compiler to be Clang, so this shouldn't effect other
builds, but in the case of our Apple cross builds, we'd end up with a
duplicated `--target=arm64-apple-darwin` on the compiler line, given we
are already setting `--target` for Darwin builds.

Would fix bitcoin#31748.

Author: fanquake

depends/Makefile

@@ -203,6 +203,7 @@ endif
 $(host_prefix)/toolchain.cmake : toolchain.cmake.in $(host_prefix)/.stamp_$(final_build_id)
 	@mkdir -p $(@D)
 	sed -e 's|@depends_crosscompiling@|$(crosscompiling)|' \
+            -e 's|@host@|$(host)|' \
             -e 's|@host_system_name@|$($(host_os)_cmake_system_name)|' \
             -e 's|@host_system_version@|$($(host_os)_cmake_system_version)|' \
             -e 's|@host_arch@|$(host_arch)|' \

depends/toolchain.cmake.in

@@ -13,6 +13,10 @@ if(@depends_crosscompiling@)
   set(CMAKE_SYSTEM_NAME @host_system_name@)
   set(CMAKE_SYSTEM_VERSION @host_system_version@)
   set(CMAKE_SYSTEM_PROCESSOR @host_arch@)
+
+  set(CMAKE_C_COMPILER_TARGET @host@)
+  set(CMAKE_CXX_COMPILER_TARGET @host@)
+  set(CMAKE_OBJCXX_COMPILER_TARGET @host@)
 endif()
 
 if(NOT DEFINED CMAKE_C_FLAGS_INIT)