go/analysis/passes/asmdecl: add build tag for loong64

The arch "loong64" doesn't exist before go 1.19 causing a "unknown
architecture" message printed to stderr when asmdecl is imported.

A build tag now prevents "loong64" from being checked in earlier
versions of go.

Fixes golang/go#52646

Change-Id: Ibc83ce02d92cfb709a773ca758b21a80516a399c
Reviewed-on: https://go-review.googlesource.com/c/tools/+/403874
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>

go/analysis/passes/asmdecl/arches_go118.go

@@ -0,0 +1,12 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build !go1.19
+// +build !go1.19
+
+package asmdecl
+
+func additionalArches() []*asmArch {
+	return nil
+}

go/analysis/passes/asmdecl/arches_go119.go

@@ -0,0 +1,14 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build go1.19
+// +build go1.19
+
+package asmdecl
+
+var asmArchLoong64 = asmArch{name: "loong64", bigEndian: false, stack: "R3", lr: true}
+
+func additionalArches() []*asmArch {
+	return []*asmArch{&asmArchLoong64}
+}

go/analysis/passes/asmdecl/asmdecl.go

@@ -86,7 +86,6 @@ var (
 	asmArchArm      = asmArch{name: "arm", bigEndian: false, stack: "R13", lr: true}
 	asmArchArm64    = asmArch{name: "arm64", bigEndian: false, stack: "RSP", lr: true, retRegs: []string{"R0", "F0"}}
 	asmArchAmd64    = asmArch{name: "amd64", bigEndian: false, stack: "SP", lr: false, retRegs: []string{"AX", "X0"}}
-	asmArchLoong64  = asmArch{name: "loong64", bigEndian: false, stack: "R3", lr: true}
 	asmArchMips     = asmArch{name: "mips", bigEndian: true, stack: "R29", lr: true}
 	asmArchMipsLE   = asmArch{name: "mipsle", bigEndian: false, stack: "R29", lr: true}
 	asmArchMips64   = asmArch{name: "mips64", bigEndian: true, stack: "R29", lr: true}
@@ -102,7 +101,6 @@ var (
 		&asmArchArm,
 		&asmArchArm64,
 		&asmArchAmd64,
-		&asmArchLoong64,
 		&asmArchMips,
 		&asmArchMipsLE,
 		&asmArchMips64,
@@ -116,6 +114,7 @@ var (
 )
 
 func init() {
+	arches = append(arches, additionalArches()...)
 	for _, arch := range arches {
 		arch.sizes = types.SizesFor("gc", arch.name)
 		if arch.sizes == nil {