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_unsupported.go

@@ -0,0 +1,28 @@
+//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
+
+// 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 {
+	return nil
+}