cmd/link, runtime: add initial cgo support for ppc64

We should be able to build docker after this get applied.

Updates #13192

Change-Id: I5378d3518fac52d6bd4c97828884c1b382b7ace5
GitHub-Last-Rev: 210b7bc2e172f641f1102982e04542bf73a1aa46
GitHub-Pull-Request: golang/go#28546
Reviewed-on: https://go-review.googlesource.com/c/146898


Reviewed-by: Jiang Ma <ma.jiang@zte.com.cn>
Reviewed-by: Clément Chigot <clement.chigot@atos.net>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>