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.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 {