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/mkpost.go

@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build ignore
-// +build ignore
 
 // mkpost processes the output of cgo -godefs to
 // modify the generated types. It is used to clean up
@@ -218,8 +217,7 @@ func main() {
 	replacement := fmt.Sprintf(`$1 | go run mkpost.go
 // Code generated by the command above; see README.md. DO NOT EDIT.
 
-//go:build %s && %s
-// +build %s,%s`, goarch, goos, goarch, goos)
+//go:build %s && %s`, goarch, goos)
 	cgoCommandRegex := regexp.MustCompile(`(cgo -godefs .*)`)
 	b = cgoCommandRegex.ReplaceAll(b, []byte(replacement))