Merge bitcoin/bitcoin#29665: build, depends: Fix libmultiprocess cross-compilation

2de2ea2 build, depends: Fix `libmultiprocess` cross-compilation (Hennadii Stepanov)

Pull request description:

  On the master branch @ 3b12fc7, the following command fails:
  ```
  $ make -C depends libmultiprocess HOST=arm64-apple-darwin MULTIPROCESS=1
  ...
  [100%] Linking CXX executable mpgen
  ...
  clang++: error: linker command failed with exit code 1 (use -v to see invocation)
  ...
  ```

  This PR prevents building all default targets that include `mpgen`, which expectedly fails to link when cross-compiling.

ACKs for top commit:
  ryanofsky:
    Code review ACK 2de2ea2
  fanquake:
    ACK 2de2ea2 - I checked that this fixes the macOS cross-compilation issue. I'm assuming these packages are also likely to change further in the (near) future, given the changes going in upstream: https://github.com/chaincodelabs/libmultiprocess/pulls?q=is%3Apr+is%3Aclosed.

Tree-SHA512: 563551afbe483c923b52c6171f9d73bcc30bc4febd821b5abfe8aadb2ac601b94c2d10a73746ace3710d9f0afa4798eb090e77ccb1ae66a819495912802d91c9

Author: fanquake

depends/packages/libmultiprocess.mk

@@ -20,7 +20,7 @@ define $(package)_config_cmds
 endef
 
 define $(package)_build_cmds
-  $(MAKE)
+  $(MAKE) multiprocess
 endef
 
 define $(package)_stage_cmds