Fix the build with gcc 4.7 and -std=c++11.

The error message was:

/home/espindola/llvm/llvm/tools/gold/gold-plugin.cpp: In function ‘ld_plugin_status cleanup_hook()’:
/home/espindola/llvm/llvm/tools/gold/gold-plugin.cpp:461:30: error: cannot pass objects of non-trivially-copyable type ‘std::string {aka class std::basic_string<char>}’ through ‘...’

I will check if this was a clang or gcc issue.