all: add missing //go:build comments

These were apparently overlooked in CL 357329, CL 294490, CL 296889,
and other various updates to this module. (I noticed them via gopls
while investigating golang/go#49466.)

Updates golang/go#41184

Change-Id: Id042bb6fe5282e6d528e9315acf2ad29d0df58ba
Reviewed-on: https://go-review.googlesource.com/c/sys/+/362577
Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>

plan9/mksyscall.go

@@ -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 ignore
 // +build ignore
 
 /*
@@ -49,7 +50,12 @@ func cmdLine() string {
 	return "go run mksyscall.go " + strings.Join(os.Args[1:], " ")
 }
 
-// buildTags returns build tags
+// goBuildTags returns build tags in the go:build format.
+func goBuildTags() string {
+	return strings.ReplaceAll(*tags, ",", " && ")
+}
+
+// plusBuildTags returns build tags in the +build format.
 func buildTags() string {
 	return *tags
 }
@@ -377,12 +383,13 @@ func main() {
 		}
 		file.Close()
 	}
-	fmt.Printf(srcTemplate, cmdLine(), buildTags(), text)
+	fmt.Printf(srcTemplate, cmdLine(), goBuildTags(), plusBuildTags(), text)
 }
 
 const srcTemplate = `// %s
 // Code generated by the command above; see README.md. DO NOT EDIT.
 
+//go:build %s
 // +build %s
 
 package plan9