Notarize toolchain.

[vgorloff]

Either codesign every binary or codesign dmg/zip/bundle.

See:

• https://stackoverflow.com/questions/57808250/osx-notarize-for-executables-not-an-app
• https://developer.apple.com/forums/thread/122045
• https://stackoverflow.com/questions/56615396/macos-notarization-and-multiple-embedded-tools-bundles
• https://developer.apple.com/documentation/security/notarizing_macos_software_before_distribution/customizing_the_notarization_workflow