liblink used to encode both SETEQ BP and SETEQ CH as 0f 94 c5,
however, SETEQ BP should have used a REX prefix.

Fixes #8545.

Change-Id: Ie59c990cdd0ec506cffe4318e9ad1b48db5e57dd
Reviewed-on: https://go-review.googlesource.com/2270Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>