refactor: use go:linkname to link internal/platform.CgoSupported

Instead of creating a custom platform package, use the go:linkname
directive to directly link to Go's standard library internal/platform.CgoSupported
function. This approach:

- Eliminates the need to maintain a copy of platform support data
- Uses Go's canonical platform information directly
- Reduces code duplication and maintenance burden
- Follows Go's linkname pattern for accessing internal packages

Changes:
- Added import _ "unsafe" to enable linkname
- Added //go:linkname directive for cgoSupported function
- Removed custom runtime/internal/lib/internal/platform package
- Updated function call from platform.CgoSupported to cgoSupported

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

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

@@ -14,8 +14,7 @@ import (
 	"runtime"
 	"strconv"
 	"strings"
-
-	"github.com/goplus/llgo/runtime/internal/lib/internal/platform"
+	_ "unsafe"
 )
 
 // Type aliases to reference standard library types
@@ -79,7 +78,7 @@ func defaultContext() Context {
 		c.CgoEnabled = false
 	default:
 		if runtime.GOARCH == c.GOARCH && runtime.GOOS == c.GOOS {
-			c.CgoEnabled = platform.CgoSupported(c.GOOS, c.GOARCH)
+			c.CgoEnabled = cgoSupported(c.GOOS, c.GOARCH)
 			break
 		}
 		c.CgoEnabled = false
@@ -122,3 +121,6 @@ func buildToolTags() []string {
 		"goexperiment.boringcrypto", // Default boring crypto experiment
 	}
 }
+
+//go:linkname cgoSupported internal/platform.CgoSupported
+func cgoSupported(goos, goarch string) bool