[native assets] `dart build` support for cross compilation from Windows/Macos to Linux

[dcharkes]

• Upstream changes needed [native_toolchain_c] Cross compile from Mac/Windows to Linux native#2185
• The Dart SDK might ship a dart-precompiled-runtime in the MacOS, it doesn't ship the C toolchain for cross compilation.
  • So, the CCompilerConfig will be null. (Passing in a CCompiler in general from dartdev is a separate issue.)