darwin: runtime support

This commit attemts to support containerd on darwin platform.  With an
external runtime shim, ctr run should work with, for instance,
--runtime=io.containerd.runu.v1.  An example of runtime and shim is
managed under different repository (github.com/ukontainer/runu/).

Signed-off-by: Hajime Tazaki <thehajime@gmail.com>

platforms/defaults_unix.go

@@ -1,5 +1,5 @@
-//go:build !windows
-// +build !windows
+//go:build !windows && !darwin
+// +build !windows,!darwin
 
 /*
    Copyright The containerd Authors.