Implement llgo build mode support (#1197)

- Add BuildMode type with three build modes: exe, c-archive, c-shared
- Restrict buildmode flag to llgo build command only (not run/install/test)
- Implement build mode specific linker arguments:
  - c-shared: use -shared -fPIC flags
  - c-archive: use ar tool to create static archive
  - exe: default executable mode
- Add normalizeOutputPath function for platform-specific file naming conventions
- Generate C header files for library modes
- Fix buildmode flag conflict by removing from PassArgs
- Add comprehensive test coverage for all build modes
- Resolve duplicate logic between defaultAppExt and normalizeOutputPath

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

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

cmd/internal/test/test.go

@@ -7,6 +7,7 @@ import (
 	"github.com/goplus/llgo/cmd/internal/base"
 	"github.com/goplus/llgo/cmd/internal/flags"
 	"github.com/goplus/llgo/internal/build"
+	"github.com/goplus/llgo/internal/mockable"
 )
 
 // llgo test
@@ -30,12 +31,15 @@ func runCmd(cmd *base.Command, args []string) {
 	}
 
 	conf := build.NewDefaultConf(build.ModeTest)
-	flags.UpdateConfig(conf)
+	if err := flags.UpdateConfig(conf); err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		mockable.Exit(1)
+	}
 
 	args = cmd.Flag.Args()
 	_, err := build.Do(args, conf)
 	if err != nil {
 		fmt.Fprintln(os.Stderr, err)
-		os.Exit(1)
+		mockable.Exit(1)
 	}
 }