code: def get_default_cross_build_toolchain(source_root_dir, opts):
      build_system_name = platform.system().lower()
      (
          target_system_name,
          target_system_arch,
      ) = opts.target_platform.split("-")
  
      default_toolchains_dir = os.path.join(
          source_root_dir, "build", "toolchains"
      )
  
      if build_system_name == "darwin":
          if target_system_name != "darwin":
              raise RuntimeError(
                  "Cross-compilation to darwin from non-darwin is not supported"
              )
          return os.path.join(
              default_toolchains_dir,
              f"cross-build.host.darwin.target.{target_system_arch}-darwin-default.clang.toolchain",
          )
      elif build_system_name == "linux":
          if target_system_name == "android":
              # use toolchain from NDK
              if opts.android_ndk_root_dir is None:
                  raise RuntimeError(
                      "Android NDK root dir not specified"
                  )
              return os.path.join(
                  opts.android_ndk_root_dir,
                  "build",
                  "cmake",
                  "android.toolchain.cmake",
              )
          elif target_system_name == "linux":
              return os.path.join(
                  default_toolchains_dir,
                  f"cross-build.host.linux.target.{target_system_arch}-linux-gnu.clang.toolchain",
              )
          else:
              raise RuntimeError(
                  f"Cross-compilation from {build_system_name} to {target_system_name} is not supported"
              )
      else:
          raise RuntimeError(
              f"Cross-compilation from {build_system_name} is not supported"
          )
  
imports: import argparse
  import copy
  import logging
  import os
  import platform
  import subprocess
  import sys
  import tempfile
  from typing import Dict
label_desc: Use image node_id 6 for calling a global function with example usage: get_default_cross_build_toolchain(source_root_dir, opts) and returns: os, os, os