[dev.typeparams] cmd/link: mangle symbol ABI name on Plan 9

It is probably not strictly necessary (as we don't support
external linking on Plan 9). Do it for consistency (and less
confusion).

Change-Id: I0b48562061273ccbd4be83db4a981b8e465b1c95
Reviewed-on: https://go-review.googlesource.com/c/go/+/321331


Trust: Cherry Mui <cherryyz@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>