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/mksyscall_aix_ppc.go

@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build ignore
-// +build ignore
 
 /*
 This program reads a file containing function prototypes
@@ -48,11 +47,6 @@ func goBuildTags() string {
 	return strings.ReplaceAll(*tags, ",", " && ")
 }
 
-// plusBuildTags returns build tags in the +build format.
-func plusBuildTags() string {
-	return *tags
-}
-
 // Param is function parameter
 type Param struct {
 	Name string
@@ -392,14 +386,13 @@ func main() {
 		imp = "import \"golang.org/x/sys/unix\"\n"
 
 	}
-	fmt.Printf(srcTemplate, cmdLine(), goBuildTags(), plusBuildTags(), pack, cExtern, imp, text)
+	fmt.Printf(srcTemplate, cmdLine(), goBuildTags(), pack, cExtern, imp, text)
 }
 
 const srcTemplate = `// %s
 // Code generated by the command above; see README.md. DO NOT EDIT.
 
 //go:build %s
-// +build %s
 
 package %s