plugin: remove go < 1.8 stub, enable on windows and arm64

- we don't support go < 1.8. this restriction as added because plugin support
  requires go 1.8 or up, but with such old versions being EOL, this check was
  rather redundant
- add back arm64 support; in 0b76334, non-amd64
  was disabled, pending golang/go#17138, which was tracking arm64 support, and
  is now resolved. It's unclear if architectures other than amd64 and arm64 are
  supported, so keeping it restricted to amd64 and arm64.
- enable plugin support on Windows; it was disabled in 69ee7f8
  but the code looks to take windows into account.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

Author: thaJeztah

plugin.go

@@ -138,7 +138,11 @@ var register = struct {
 	r []*Registration
 }{}
 
-// Load loads all plugins at the provided path into containerd
+// Load loads all plugins at the provided path into containerd.
+//
+// Load is currently only implemented on non-static, non-gccgo builds for amd64
+// and arm64, and plugins must be built with the exact same version of Go as
+// containerd itself.
 func Load(path string) (err error) {
 	defer func() {
 		if v := recover(); v != nil {

plugin_go18.go plugin_supported.goplugin_go18.go renamed to plugin_supported.go

@@ -1,4 +1,4 @@
-//go:build go1.8 && !windows && amd64 && !static_build && !gccgo
+//go:build (amd64 || arm64) && !static_build && !gccgo
 
 /*
    Copyright The containerd Authors.

plugin_other.go plugin_unsupported.goplugin_other.go renamed to plugin_unsupported.go

@@ -1,4 +1,4 @@
-//go:build !go1.8 || windows || !amd64 || static_build || gccgo
+//go:build (!amd64 && !arm64) || static_build || gccgo
 
 /*
    Copyright The containerd Authors.
@@ -18,7 +18,11 @@
 
 package plugin
 
+// loadPlugins is not supported;
+//
+// - with gccgo: gccgo has no plugin support golang/go#36403
+// - on static builds; https://github.com/containerd/containerd/commit/0d682e24a1ba8e93e5e54a73d64f7d256f87492f
+// - on architectures other than amd64 and arm64 (other architectures need to be tested)
 func loadPlugins(path string) error {
-	// plugins not supported until 1.8
 	return nil
 }