message: build: use aarch64 Clang if cross-compiling for darwin on aarch64
  If we're cross-compiling for darwin on aarch64 hardware, we need need to
  use a Clang that will run on that hardware.
  Only tested in a Linux Docker container (aarch64-unknown-linux-gnu),
  running on an Apple M1 mac-mini (aarch64-apple-darwin20.5.0).
diff: package=native_clang
  $(package)_version=10.0.1
  $(package)_download_path=https://github.com/llvm/llvm-project/releases/download/llvmorg-$($(package)_version)
  +ifneq (,$(findstring aarch64,$(BUILD)))
  +$(package)_download_file=clang+llvm-$($(package)_version)-aarch64-linux-gnu.tar.xz
  +$(package)_file_name=clang+llvm-$($(package)_version)-aarch64-linux-gnu.tar.xz
  +$(package)_sha256_hash=90dc69a4758ca15cd0ffa45d07fbf5bf4309d47d2c7745a9f0735ecffde9c31f
  +else
  $(package)_download_file=clang+llvm-$($(package)_version)-x86_64-linux-gnu-ubuntu-16.04.tar.xz
  $(package)_file_name=clang+llvm-$($(package)_version)-x86_64-linux-gnu-ubuntu-16.04.tar.xz
  $(package)_sha256_hash=48b83ef827ac2c213d5b64f5ad7ed082c8bcb712b46644e0dc5045c6f462c231
  +endif
  define $(package)_preprocess_cmds
  rm -f $($(package)_extract_dir)/lib/libc++abi.so*
  