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>

Author: Jorropo

src/runtime/os_linux_futex32.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build linux && (386 || arm || mips || mipsle || ppc)
+//go:build linux && (386 || arm || mips || mipsle)
 
 package runtime
 

src/runtime/os_linux_futex64.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build linux && !(386 || arm || mips || mipsle || ppc || s390)
+//go:build linux && !(386 || arm || mips || mipsle)
 
 package runtime