This reverts commit 3607c5f4.

This was causing failures on amd64 machines without AVX.

Fixes #16939

Change-Id: I70080fbb4e7ae791857334f2bffd847d08cb25fa
Reviewed-on: https://go-review.googlesource.com/28274Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>