runtime: clean dead architectures from go:build constraint

runtime: clean dead architectures from go:build constraint

I've didn't caught theses while reviewing CL 701615.

Change-Id: I721978c173a255eb6d7c3e43dea2b903a9fd016d
Reviewed-on: https://go-review.googlesource.com/c/go/+/712740
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Jorropo <jorropo.pgm@gmail.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: David Chase <drchase@google.com>