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>
diff --git a/cpu/asm_aix_ppc64.s b/cpu/asm_aix_ppc64.s
index 6b4027b..db9171c 100644
--- a/cpu/asm_aix_ppc64.s
+++ b/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"
diff --git a/cpu/cpu_arm64.s b/cpu/cpu_arm64.s
index cfc08c9..c61f95a 100644
--- a/cpu/cpu_arm64.s
+++ b/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"
diff --git a/cpu/cpu_s390x.s b/cpu/cpu_s390x.s
index 964946d..96f81e2 100644
--- a/cpu/cpu_s390x.s
+++ b/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"
diff --git a/cpu/cpu_x86.s b/cpu/cpu_x86.s
index 2f557a5..39acab2 100644
--- a/cpu/cpu_x86.s
+++ b/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