feat: restrict //export different symbol names to baremetal targets only

- Modified cl/import.go to check LLGO_TARGET_BAREMETAL env var
- For baremetal targets: //export SymbolName allows different export name (TinyGo-style)
- For normal targets: //export SymbolName uses SymbolName as function name (standard Go)
- Set LLGO_TARGET_BAREMETAL=1 in internal/build/build.go when baremetal tag present
- Added test for normal targets in _demo/normal/export/
- Added CI test to verify both embedded and normal target behavior

Generated with [codeagent](https://github.com/qbox/codeagent)
Co-authored-by: luoliwoshang <51194195+luoliwoshang@users.noreply.github.com>

internal/build/build.go

@@ -222,6 +222,13 @@ func Do(args []string, conf *Config) ([]Package, error) {
 	}
 	if len(export.BuildTags) > 0 {
 		tags += "," + strings.Join(export.BuildTags, ",")
+		// Set environment variable if building for baremetal target
+		for _, tag := range export.BuildTags {
+			if tag == "baremetal" {
+				os.Setenv("LLGO_TARGET_BAREMETAL", "1")
+				break
+			}
+		}
 	}
 	cfg := &packages.Config{
 		Mode:       loadSyntax | packages.NeedDeps | packages.NeedModule | packages.NeedExportFile,