cmd/link: set canUsePlugins only on platforms that support plugin
If a program imports the plugin package, but the plugin build mode
is not supported on the platform, it still cannot use plugin. Don't
treat it like so.
Updates #76815.
Change-Id: I0fd719427d7c3cc96a94ce1f92d6e4457da92a90
Reviewed-on: https://go-review.googlesource.com/c/go/+/730164 Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>