all: update go directive to 1.18

Done with:

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

Using go1.21.3.

Also delete the build constraints that are always satisfied
when using supported Go versions.

For golang/go#60268.

Change-Id: Iab4a7237a368b1ac05bb72a646501defb51503f1
Reviewed-on: https://go-review.googlesource.com/c/arch/+/534197
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>

arm/armspec/spec.go

@@ -2,10 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build go1.6 && (!386 || go1.8)
-// +build go1.6
-// +build !386 go1.8
-
 // ... see golang.org/issue/12840
 
 // Armspec reads the “ARM Architecture Reference Manual”

arm/armspec/specmap.go

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

arm64/arm64spec/spec.go

@@ -2,9 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build (go1.6 && amd64) || go1.8
-// +build go1.6,amd64 go1.8
-
 // arm64spec reads the ``ARMv8-A Reference Manual''
 // to collect instruction encoding details and writes those
 // details to standard output in JSON format.

go.mod

@@ -1,5 +1,5 @@
 module golang.org/x/arch
 
-go 1.17
+go 1.18
 
 require rsc.io/pdf v0.1.1

ppc64/ppc64spec/spec.go

@@ -2,9 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build (go1.6 && amd64) || go1.8
-// +build go1.6,amd64 go1.8
-
 // Power64spec reads the “Power ISA V2.07” Manual
 // to collect instruction encoding details and writes those details to standard output
 // in CSV format.

ppc64/ppc64util/util.go

@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build ignore
-// +build ignore
 
 // Generate interesting test cases from ppc64 objdump via
 // go run util.go