This change allows to cross compile darwin binaries on Linux and
eliminate the need to maintain macOS special infrastructure.

This change uses https://github.com/tpoechtrager/osxcross, a fairly
common cross compilation toolchain script.  It is used by
[Docker](https://github.com/docker/golang-cross).  So far in my testing,
it looks good.

Alternatively, we could also compile without CGO.  Apparently that
works, as the primary cgo libraries (e.g. runc/docker/nvidia) aren't
applicable in darwin environment.  I chose against this approach, as
some golang standard libraries (e.g. net, os) use cgo and I feared that
we'd introduce a subtle change - I got bitten by subtle differences
between cgo and non-cgo DNS lookup behavior before for example.