cmd/link: don't generate runtime.text twice for buildmode=plugin on darwin

https://golang.org/cl/29394 changed to include runtime.text and
runtime.etext in ctxt.Textp as a work around.
But it seems that the CL forgot to change genasmsym.
As a result, we are generating runtime.text and runtime.etext twice.

Change-Id: If7f8faf496c1c489ffa4804da712f91a3d3f4be4
Reviewed-on: https://go-review.googlesource.com/62810


Reviewed-by: David Crawshaw <crawshaw@golang.org>
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>