all: update go directive to 1.18

Done with:

go get [email protected]
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 <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Auto-Submit: Dmitri Shuralyov <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: dmitshur

cpu/asm_aix_ppc64.s

@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build gc
-// +build gc
 
 #include "textflag.h"
 

cpu/cpu_aix.go

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

cpu/cpu_arm64.s

@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build gc
-// +build gc
 
 #include "textflag.h"
 

cpu/cpu_gc_arm64.go

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

cpu/cpu_gc_s390x.go

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

cpu/cpu_gc_x86.go

@@ -3,8 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build (386 || amd64 || amd64p32) && gc
-// +build 386 amd64 amd64p32
-// +build gc
 
 package cpu
 

cpu/cpu_gccgo_arm64.go

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

cpu/cpu_gccgo_s390x.go

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

cpu/cpu_gccgo_x86.c

@@ -3,8 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build (386 || amd64 || amd64p32) && gccgo
-// +build 386 amd64 amd64p32
-// +build gccgo
 
 #include <cpuid.h>
 #include <stdint.h>

cpu/cpu_gccgo_x86.go

@@ -3,8 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build (386 || amd64 || amd64p32) && gccgo
-// +build 386 amd64 amd64p32
-// +build gccgo
 
 package cpu