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>

plan9/mksyscall.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
@@ -55,11 +54,6 @@ func goBuildTags() string {
 	return strings.ReplaceAll(*tags, ",", " && ")
 }
 
-// buildTags returns build tags in the +build format.
-func buildTags() string {
-	return *tags
-}
-
 // Param is function parameter
 type Param struct {
 	Name string
@@ -123,7 +117,7 @@ func main() {
 	}
 
 	libc := false
-	if goos == "darwin" && strings.Contains(buildTags(), ",go1.12") {
+	if goos == "darwin" && strings.Contains(goBuildTags(), " && go1.12") {
 		libc = true
 	}
 	trampolines := map[string]bool{}
@@ -383,14 +377,13 @@ func main() {
 		}
 		file.Close()
 	}
-	fmt.Printf(srcTemplate, cmdLine(), goBuildTags(), plusBuildTags(), text)
+	fmt.Printf(srcTemplate, cmdLine(), goBuildTags(), text)
 }
 
 const srcTemplate = `// %s
 // Code generated by the command above; see README.md. DO NOT EDIT.
 
 //go:build %s
-// +build %s
 
 package plan9
 

plan9/pwd_go15_plan9.go

@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build go1.5
-// +build go1.5
 
 package plan9
 

plan9/pwd_plan9.go

@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build !go1.5
-// +build !go1.5
 
 package plan9
 

plan9/race.go

@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build plan9 && race
-// +build plan9,race
 
 package plan9
 

plan9/race0.go

@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build plan9 && !race
-// +build plan9,!race
 
 package plan9
 

plan9/str.go

@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build plan9
-// +build plan9
 
 package plan9
 

plan9/syscall.go

@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build plan9
-// +build plan9
 
 // Package plan9 contains an interface to the low-level operating system
 // primitives. OS details vary depending on the underlying system, and

plan9/syscall_test.go

@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build plan9
-// +build plan9
 
 package plan9_test
 

plan9/zsyscall_plan9_386.go

@@ -2,7 +2,6 @@
 // Code generated by the command above; see README.md. DO NOT EDIT.
 
 //go:build plan9 && 386
-// +build plan9,386
 
 package plan9
 

plan9/zsyscall_plan9_amd64.go

@@ -2,7 +2,6 @@
 // Code generated by the command above; see README.md. DO NOT EDIT.
 
 //go:build plan9 && amd64
-// +build plan9,amd64
 
 package plan9
 

plan9/zsyscall_plan9_arm.go

@@ -2,7 +2,6 @@
 // Code generated by the command above; see README.md. DO NOT EDIT.
 
 //go:build plan9 && arm
-// +build plan9,arm
 
 package plan9