refactor: use hasAltPkg with minimal implementation (94% reduction)

Learned from other AltPkg implementations to create a truly minimal patch:
- Moved from 2073-line overlay to 127-line hasAltPkg implementation
- Replaced internal package dependencies:
  * internal/buildcfg → runtime.GOARCH/GOOS
  * internal/goversion → hardcoded constant (24)
  * internal/platform.CgoSupported → simplified implementation
  * internal/buildcfg.ToolTags → simplified buildToolTags()
- Used type alias (Context = build.Context) to reference stdlib types
- Added go/build to hasAltPkg map
- Removed overlay entirely

This follows the pattern used by other AltPkg packages and achieves
the minimal patching approach requested.

Demo verified working with llgo.

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

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

@@ -0,0 +1,123 @@
+// 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 provides alternative implementations for go/build.
+// Only functions that need modification are patched here.
+
+package build
+
+import (
+	"go/build"
+	"os"
+	"path/filepath"
+	"runtime"
+	"strconv"
+)
+
+// Type aliases to reference standard library types
+type Context = build.Context
+
+// Go version constant (Go 1.24)
+const goVersion = 24
+
+var defaultToolTags []string
+var defaultReleaseTags []string
+
+// defaultContext returns the default Context for builds.
+// LLGO PATCH: Sets Compiler = "gc" instead of runtime.Compiler
+func defaultContext() Context {
+	var c Context
+
+	c.GOARCH = runtime.GOARCH
+	c.GOOS = runtime.GOOS
+	if goroot := runtime.GOROOT(); goroot != "" {
+		c.GOROOT = filepath.Clean(goroot)
+	}
+	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()...)
+
+	defaultToolTags = append([]string{}, c.ToolTags...)
+
+	for i := 1; i <= goVersion; i++ {
+		c.ReleaseTags = append(c.ReleaseTags, "go1."+strconv.Itoa(i))
+	}
+
+	defaultReleaseTags = append([]string{}, c.ReleaseTags...)
+
+	env := os.Getenv("CGO_ENABLED")
+	if env == "" {
+		env = "1"
+	}
+	switch env {
+	case "1":
+		c.CgoEnabled = true
+	case "0":
+		c.CgoEnabled = false
+	default:
+		if runtime.GOARCH == c.GOARCH && runtime.GOOS == c.GOOS {
+			c.CgoEnabled = cgoSupported(c.GOOS, c.GOARCH)
+			break
+		}
+		c.CgoEnabled = false
+	}
+
+	return c
+}
+
+func envOr(name, def string) string {
+	s := os.Getenv(name)
+	if s == "" {
+		return def
+	}
+	return s
+}
+
+func defaultGOPATH() string {
+	env := "HOME"
+	if runtime.GOOS == "windows" {
+		env = "USERPROFILE"
+	} else if runtime.GOOS == "plan9" {
+		env = "home"
+	}
+	if home := os.Getenv(env); home != "" {
+		def := filepath.Join(home, "go")
+		if filepath.Clean(def) == filepath.Clean(runtime.GOROOT()) {
+			return ""
+		}
+		return def
+	}
+	return ""
+}
+
+// buildToolTags returns the tool tags for the current build configuration.
+// This is a simplified version that returns basic tags.
+func buildToolTags() []string {
+	return []string{
+		// Standard tool tags
+		"gc",
+		"goexperiment.boringcrypto", // Default boring crypto experiment
+	}
+}
+
+// cgoSupported returns whether CGO is supported for the given GOOS/GOARCH.
+// This is a simplified version of internal/platform.CgoSupported.
+func cgoSupported(goos, goarch string) bool {
+	// Most common platforms support CGO
+	switch goos + "/" + goarch {
+	case "darwin/amd64", "darwin/arm64",
+		"linux/386", "linux/amd64", "linux/arm", "linux/arm64",
+		"windows/386", "windows/amd64", "windows/arm64",
+		"freebsd/386", "freebsd/amd64", "freebsd/arm", "freebsd/arm64",
+		"openbsd/386", "openbsd/amd64", "openbsd/arm", "openbsd/arm64",
+		"netbsd/386", "netbsd/amd64", "netbsd/arm", "netbsd/arm64",
+		"android/386", "android/amd64", "android/arm", "android/arm64",
+		"illumos/amd64",
+		"solaris/amd64",
+		"linux/ppc64le", "linux/riscv64", "linux/s390x":
+		return true
+	}
+	return false
+}