cmd/link: force external linking for plugins

Fixes #17415

Change-Id: I6f896d549092e5e0dba72351e5385992b4cbe90f
Reviewed-on: https://go-review.googlesource.com/30933
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/link/internal/ld/config.go

@@ -201,6 +201,8 @@ func mustLinkExternal(ctxt *Link) (res bool, reason string) {
 			// Internal linking does not support TLS_IE.
 			return true, "buildmode=pie"
 		}
+	case BuildmodePlugin:
+		return true, "buildmode=plugin"
 	case BuildmodeShared:
 		return true, "buildmode=shared"
 	}