libatf-c++: Nix expsym for now.

Evidently this is machine-dependent owing to variation in C++ symbols
across ABIs:

- amd64 has _ZN3atf4text7to_typeIlEET_RKNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEE,
  i.e,
  long atf::text::to_type<long>(std::__cxx11::basic_string<char,
  std::char_traits<char>, std::allocator<char> > const&)

- i386 has _ZN3atf4text7to_typeIxEET_RKNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEE,
  i.e.,
  long long atf::text::to_type<long long>(std::__cxx11::basic_string<char,
  std::char_traits<char>, std::allocator<char> > const&)

Thanks, C++...

PR lib/58838: shared libraries in base should all have expsym lists