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 6bd0710831, 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 0b44e24c07
  but the code looks to take windows into account.

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

plugin/plugin_supported.go

@@ -0,0 +1,62 @@
+//go:build (amd64 || arm64) && !static_build && !gccgo
+
+/*
+   Copyright The containerd Authors.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+package plugin
+
+import (
+	"fmt"
+	"path/filepath"
+	"plugin"
+	"runtime"
+)
+
+// loadPlugins loads all plugins for the OS and Arch
+// that containerd is built for inside the provided path
+func loadPlugins(path string) error {
+	abs, err := filepath.Abs(path)
+	if err != nil {
+		return err
+	}
+	pattern := filepath.Join(abs, fmt.Sprintf(
+		"*-%s-%s.%s",
+		runtime.GOOS,
+		runtime.GOARCH,
+		getLibExt(),
+	))
+	libs, err := filepath.Glob(pattern)
+	if err != nil {
+		return err
+	}
+	for _, lib := range libs {
+		if _, err := plugin.Open(lib); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+// getLibExt returns a platform specific lib extension for
+// the platform that containerd is running on
+func getLibExt() string {
+	switch runtime.GOOS {
+	case "windows":
+		return "dll"
+	default:
+		return "so"
+	}
+}