all: update go directive to 1.18

Done with:

go get go@1.18
go mod tidy
go fix ./...

Using go1.21.3.

Also update code generators to use only the new go:build lines,
not the old +build ones.

For golang/go#60268.

Change-Id: I6aabc42efb6ab3329981100e1db2263aac5e92a6
Reviewed-on: https://go-review.googlesource.com/c/sys/+/534222
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

unix/mksysnum.go

@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build ignore
-// +build ignore
 
 // Generate system call table for DragonFly, NetBSD,
 // FreeBSD or OpenBSD from master list (for example,
@@ -34,11 +33,6 @@ func goBuildTags() string {
 	return fmt.Sprintf("%s && %s", goarch, goos)
 }
 
-// plusBuildTags returns build tags in the +build format.
-func plusBuildTags() string {
-	return fmt.Sprintf("%s,%s", goarch, goos)
-}
-
 func checkErr(err error) {
 	if err != nil {
 		fmt.Fprintf(os.Stderr, "%v\n", err)
@@ -182,14 +176,13 @@ func main() {
 	err := s.Err()
 	checkErr(err)
 
-	fmt.Printf(template, cmdLine(), goBuildTags(), plusBuildTags(), text)
+	fmt.Printf(template, cmdLine(), goBuildTags(), text)
 }
 
 const template = `// %s
 // Code generated by the command above; see README.md. DO NOT EDIT.
 
 //go:build %s
-// +build %s
 
 package unix