refactor: follow Go stdlib conventions for build.go

- Created internal/buildcfg package with DefaultCGO_ENABLED constant
- Updated CGO_ENABLED handling to use buildcfg.DefaultCGO_ENABLED
- Added original Go stdlib comment for release tags
- Simplified toolTags usage by using it directly instead of wrapper function

🤖 Generated with [codeagent](https://github.com/qbox/codeagent)
Co-authored-by: luoliwoshang <luoliwoshang@users.noreply.github.com>

runtime/internal/lib/go/build/build.go

@@ -15,6 +15,8 @@ import (
 	"strconv"
 	"strings"
 	_ "unsafe"
+
+	"github.com/goplus/llgo/runtime/internal/lib/internal/buildcfg"
 )
 
 // Type aliases to reference standard library types
@@ -77,10 +79,13 @@ func defaultContext() Context {
 	c.GOPATH = envOr("GOPATH", defaultGOPATH())
 	// LLGO PATCH: Use "gc" instead of runtime.Compiler to avoid "unknown compiler" error
 	c.Compiler = "gc"
-	c.ToolTags = append(c.ToolTags, buildToolTags()...)
+	c.ToolTags = append(c.ToolTags, toolTags...)
 
 	defaultToolTags = append([]string{}, c.ToolTags...)
 
+	// Each major Go release in the Go 1.x series adds a new
+	// "go1.x" release tag. That is, the go1.x tag is present in
+	// all releases >= Go 1.x.
 	goVersion := parseGoVersion()
 	for i := 1; i <= goVersion; i++ {
 		c.ReleaseTags = append(c.ReleaseTags, "go1."+strconv.Itoa(i))
@@ -90,7 +95,7 @@ func defaultContext() Context {
 
 	env := os.Getenv("CGO_ENABLED")
 	if env == "" {
-		env = "1"
+		env = buildcfg.DefaultCGO_ENABLED
 	}
 	switch env {
 	case "1":
@@ -133,7 +138,3 @@ func defaultGOPATH() string {
 	return ""
 }
 
-// buildToolTags returns the tool tags for the current build configuration.
-func buildToolTags() []string {
-	return toolTags
-}

runtime/internal/lib/internal/buildcfg/buildcfg.go

@@ -0,0 +1,12 @@
+// Copyright 2024 The GoPlus Authors (goplus.org). All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package buildcfg
+
+// llgo:skipall
+type _buildcfg struct{}
+
+// DefaultCGO_ENABLED is the default value for CGO_ENABLED
+// when the environment variable is not set.
+const DefaultCGO_ENABLED = "1"