_Lock_at_thread_exit_mutex() and _Unlock_at_thread_exit_mutex() are separately compiled, but aren't used outside the STL's DLL (or static LIB):

Since they aren't exported, I believe that it would be safe (i.e. binary-compatible) to mark them as noexcept, and give them C++ linkage.

It's also possible that there's no point in doing so; because they're extern "C" and we compile with /EHsc, they should already be treated as noexcept.