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/base/pass.go

@@ -75,7 +75,7 @@ func PassBuildFlags(cmd *Command) *PassArgs {
 	p.Bool("a")
 	p.Bool("linkshared", "race", "msan", "asan",
 		"trimpath", "work")
-	p.Var("p", "asmflags", "compiler", "buildmode",
+	p.Var("p", "asmflags", "compiler",
 		"gcflags", "gccgoflags", "installsuffix",
 		"ldflags", "pkgdir", "toolexec", "buildvcs")
 	return p