[release-branch.go1.13] runtime: fix wrong offset when calling ppc64x nanotime syscall

There is a wrong offset when getting the results of a clock_gettime
syscall. Although the syscall will never be called in native ppc64x,
QEMU doesn't implement VDSO, so it will return wrong values.

For #36592
Fixes #38236

Change-Id: Icf838075228dcdd62cf2c1279aa983e5993d66ee
Reviewed-on: https://go-review.googlesource.com/c/go/+/215397


Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
(cherry picked from commit 71239b4f)
Reviewed-on: https://go-review.googlesource.com/c/go/+/227179


Reviewed-by: Carlos Eduardo Seo <cseo@linux.vnet.ibm.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>