This function is passed as the at_exit parameter to
libcpp_tls_create. This parameter is marked with the
_LIBCPP_TLS_DESTRUCTOR_CC attribute. The macro is empty for pthread,
and set to stdcall for other threading models. stdcall is in turn
emptry for most cases. That's likely why this doesn't fail now.
The one exception is Windows on i686 with the Win32 threading API, where
the stdcall calling convention is used.