cmd/vet: fix assembly test files

They should be build-tagged for vet_test not ignore,
and not have a Go package clause.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/8016047

src/cmd/vet/test_asm1.s

@@ -3,9 +3,7 @@
 // license that can be found in the LICENSE file.
 
 // +build amd64
-// +build ignore
-
-package main
+// +build vet_test
 
 TEXT ·arg1(SB),0,$0-2
 	MOVB	x+0(FP), AX

src/cmd/vet/test_asm2.s

@@ -3,9 +3,7 @@
 // license that can be found in the LICENSE file.
 
 // +build 386
-// +build ignore
-
-package main
+// +build vet_test
 
 TEXT ·arg1(SB),0,$0-2
 	MOVB	x+0(FP), AX

src/cmd/vet/test_asm3.s

@@ -3,9 +3,7 @@
 // license that can be found in the LICENSE file.
 
 // +build arm
-// +build ignore
-
-package main
+// +build vet_test
 
 TEXT ·arg1(SB),0,$0-2
 	MOVB	x+0(FP), AX