[core] Garbage collect unused sections in libCling

After inlining and hiding symbols, some of them are not referenced
anymore and can be collected by the linker, reducing the library
size from 120 MB to 105 MB in a Release configuration with GCC.

Author: hahnjo

core/metacling/src/CMakeLists.txt

@@ -229,8 +229,13 @@ if(NOT APPLE AND NOT MSVC)
   # and by ALICE in https://github.com/root-project/root/issues/19889
   # Only needed for Linux: Mac uses linker namespaces and  Windows explicit export/import
   target_link_libraries(Cling PRIVATE -Wl,-Bsymbolic)
+  # We compile with -fvisibility=hidden in interpreter/, but some symbols of
+  # instantiated templates remain. Use a version script to hide them.
   target_link_libraries(Cling PRIVATE -Wl,--version-script=${CMAKE_CURRENT_SOURCE_DIR}/libCling.script)
   set_target_properties(Cling PROPERTIES LINK_DEPENDS ${CMAKE_CURRENT_SOURCE_DIR}/libCling.script)
+  # After inlining and hiding symbols, some of them are not referenced anymore
+  # and can be collected by the linker, reducing the library size.
+  target_link_libraries(Cling PRIVATE -Wl,--gc-sections)
 endif()
 
 if (CMAKE_SYSTEM_NAME MATCHES FreeBSD)