refactor: address final review comments

- Created zcgo.go with defaultCGO_ENABLED constant (Go 1.24 pattern)
- Updated parseGoVersion to panic on unexpected input instead of fallback
- Added runtime.(*Func).Name stub to symtab.go for Go 1.21/1.22 compatibility
- Removed go1.23 build constraint from demo
- Removed internal/buildcfg package (replaced by zcgo.go)

These changes ensure the demo works across Go 1.21+ versions.

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

_demo/go/gobuild/demo.go

@@ -1,5 +1,3 @@
-//go:build go1.23
-
 package main
 
 import (

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

@@ -15,8 +15,6 @@ import (
 	"strconv"
 	"strings"
 	_ "unsafe"
-
-	"github.com/goplus/llgo/runtime/internal/lib/internal/buildcfg"
 )
 
 // Type aliases to reference standard library types
@@ -63,7 +61,7 @@ func parseGoVersion() int {
 			return minor
 		}
 	}
-	return 24
+	panic("parseGoVersion: cannot parse go version from runtime.Version(): " + runtime.Version())
 }
 
 // defaultContext returns the default Context for builds.
@@ -95,7 +93,7 @@ func defaultContext() Context {
 
 	env := os.Getenv("CGO_ENABLED")
 	if env == "" {
-		env = buildcfg.DefaultCGO_ENABLED
+		env = defaultCGO_ENABLED
 	}
 	switch env {
 	case "1":

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

@@ -0,0 +1,7 @@
+// 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 build
+
+const defaultCGO_ENABLED = "1"

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

@@ -1,12 +0,0 @@
-// 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"

runtime/internal/lib/runtime/symtab.go

@@ -140,6 +140,10 @@ type Func struct {
 	opaque struct{} // unexported field to disallow conversions
 }
 
+func (f *Func) Name() string {
+	panic("todo")
+}
+
 // moduledata records information about the layout of the executable
 // image. It is written by the linker. Any changes here must be
 // matched changes to the code in cmd/link/internal/ld/symtab.go:symtab.