[release-branch.go1.8] cmd/link: put plt stubs first in Textp on ppc64x

[release-branch.go1.8] cmd/link: put plt stubs first in Textp on ppc64x

Previously call stubs were generated and inserted in
Textp after much of the text, resulting in calls too
far in some cases. This puts the call stubs first, which
in many cases makes some calls not so far, but also
enables trampolines to be generated when necessary.

This is a backport for go 1.8 based on CL38131.

Fixes #19578

Change-Id: If3ba3d5222a7f7969ed2de1df4854a1b4a80a0f0
Reviewed-on: https://go-review.googlesource.com/38472
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>