feat(crosscompile): add ESP Clang multi-platform support

- Add ESP Clang download and extraction for cross-compilation
- Support multiple platforms: darwin/amd64, darwin/arm64, linux/amd64, linux/arm64, windows/amd64
- Integrate ESP Clang with target-based configuration system
- Add ClangRoot and ClangBinPath fields to Export struct
- Support .tar.xz extraction for ESP Clang packages
- Prioritize LLGoROOT clang installation over cached downloads
- Update build system to use custom clang for embedded platforms

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

internal/build/build.go

@@ -151,7 +151,7 @@ func Do(args []string, conf *Config) ([]Package, error) {
 		conf.Goarch = runtime.GOARCH
 	}
 	// Handle crosscompile configuration first to set correct GOOS/GOARCH
-	export, err := crosscompile.UseWithTarget(conf.Goos, conf.Goarch, IsWasiThreadsEnabled(), conf.Target)
+	export, err := crosscompile.Use(conf.Goos, conf.Goarch, IsWasiThreadsEnabled(), conf.Target)
 	if err != nil {
 		return nil, fmt.Errorf("failed to setup crosscompile: %w", err)
 	}
@@ -385,7 +385,7 @@ func (c *context) compiler() *clang.Cmd {
 }
 
 func (c *context) linker() *clang.Cmd {
-	cmd := c.env.Clang()
+	cmd := c.compiler()
 	if c.crossCompile.Linker != "" {
 		cmd = clang.New(c.crossCompile.Linker)
 	}