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>

.github/workflows/llgo.yml

@@ -140,6 +140,13 @@ jobs:
           cd _demo/embed/export
           bash ../../../.github/workflows/export_test.sh
 
+      - name: Test export symbol names for normal targets
+        run: |
+          echo "Testing //export behavior for normal (non-embedded) targets..."
+          chmod +x .github/workflows/export_test_normal.sh
+          cd _demo/normal/export
+          bash ../../../.github/workflows/export_test_normal.sh
+
       - name: _xtool build tests
         run: |
           cd _xtool