VEX encoded instructions don't have a REX byte, so for PC relative
addressing we don't need to recalculate relocation offset.

Fixes #19518

Change-Id: Icf5414962de4350d76fd220817498337f90614fc
Reviewed-on: https://go-review.googlesource.com/38138
Run-TryBot: Ilya Tocar <ilya.tocar@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>