cmd/internal/obj: follow convention for generated code comment

Follow the convertion (https://golang.org/s/generatedcode) for generated
code in stringer.go.

Change-Id: I7b5fbb04ba03e8ac77a9a0a402088669469de858
Reviewed-on: https://go-review.googlesource.com/122015
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/internal/obj/arm/anames.go

-// Generated by stringer -i a.out.go -o anames.go -p arm
-// Do not edit.
+// Code generated by stringer -i a.out.go -o anames.go -p arm; DO NOT EDIT.
 
 package arm
 

src/cmd/internal/obj/arm64/anames.go

-// Generated by stringer -i a.out.go -o anames.go -p arm64
-// Do not edit.
+// Code generated by stringer -i a.out.go -o anames.go -p arm64; DO NOT EDIT.
 
 package arm64
 

src/cmd/internal/obj/mips/anames.go

-// Generated by stringer -i a.out.go -o anames.go -p mips
-// Do not edit.
+// Code generated by stringer -i a.out.go -o anames.go -p mips; DO NOT EDIT.
 
 package mips
 

src/cmd/internal/obj/ppc64/anames.go

-// Generated by stringer -i a.out.go -o anames.go -p ppc64
-// Do not edit.
+// Code generated by stringer -i a.out.go -o anames.go -p ppc64; DO NOT EDIT.
 
 package ppc64
 

src/cmd/internal/obj/s390x/anames.go

-// Generated by stringer -i a.out.go -o anames.go -p s390x
-// Do not edit.
+// Code generated by stringer -i a.out.go -o anames.go -p s390x; DO NOT EDIT.
 
 package s390x
 

src/cmd/internal/obj/stringer.go

@@ -93,8 +93,7 @@ func main() {
 	}
 }
 
-const header = `// Generated by stringer -i %s -o %s -p %s
-// Do not edit.
+const header = `// Code generated by stringer -i %s -o %s -p %s; DO NOT EDIT.
 
 package %s
 

src/cmd/internal/obj/wasm/anames.go

-// Generated by stringer -i a.out.go -o anames.go -p wasm
-// Do not edit.
+// Code generated by stringer -i a.out.go -o anames.go -p wasm; DO NOT EDIT.
 
 package wasm
 

src/cmd/internal/obj/x86/anames.go

-// Generated by stringer -i aenum.go -o anames.go -p x86
-// Do not edit.
+// Code generated by stringer -i aenum.go -o anames.go -p x86; DO NOT EDIT.
 
 package x86