cmd/link: fix minor bug in trampoline insertion

Fix a minor bug where it should use Textp2 instead of Textp. This
doesn't affect correctness. It just made the pre-allocation less
effective.

Change-Id: Ib3fa8ab3c64037e3582933970d051f278286353b
Reviewed-on: https://go-review.googlesource.com/c/go/+/232837


Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Jeremy Faller <jeremy@golang.org>