Support macOS targets

[Timmmm]

Difficult, but apparently it can be done. See the discussion here and the linked repo.

[Dylan-DPC-zz]

An attempt was made in #131

[Alexhuszagh]

This was attempted in #480, however, since we cannot distribute the generated image with macOS SDK, we've provided a Dockerfile and instructions how to build the required image for Apple targets (Darwin or iOS) on cross-toolchains.