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/build/build.go

@@ -39,6 +39,7 @@ func init() {
 
 	flags.AddCommonFlags(&Cmd.Flag)
 	flags.AddBuildFlags(&Cmd.Flag)
+	flags.AddBuildModeFlags(&Cmd.Flag)
 	flags.AddEmulatorFlags(&Cmd.Flag)
 	flags.AddEmbeddedFlags(&Cmd.Flag)
 	flags.AddOutputFlags(&Cmd.Flag)
@@ -51,7 +52,10 @@ func runCmd(cmd *base.Command, args []string) {
 	}
 
 	conf := build.NewDefaultConf(build.ModeBuild)
-	flags.UpdateConfig(conf)
+	if err := flags.UpdateBuildConfig(conf); err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		mockable.Exit(1)
+	}
 
 	args = cmd.Flag.Args()