[CMake] add_llvm_symbol_exports: Use ${native_export_file} instead of equivalent constant "symbol.*', since it is defined.
[CMake] add_llvm_symbol_exports: Use unique name for each target.
[CMake] Fix add_llvm_loadble_module.

Thanks to Edward-san, to let me know.
Fix mis-merging in AddLLVM.cmake. Sorry.