cpu: add //go:build lines to assembly files

Change-Id: I44c0bab1cc8a3b7ce59a1c690cb22046bca32bac
Reviewed-on: https://go-review.googlesource.com/c/sys/+/313689
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>

cpu/asm_aix_ppc64.s

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build gc
 // +build gc
 
 #include "textflag.h"

cpu/cpu_arm64.s

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build gc
 // +build gc
 
 #include "textflag.h"

cpu/cpu_s390x.s

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build gc
 // +build gc
 
 #include "textflag.h"

cpu/cpu_x86.s

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build (386 || amd64 || amd64p32) && gc
 // +build 386 amd64 amd64p32
 // +build gc