[v3] make check-abi

[Benjamin Kosnik]

> Appears --disable-thread (all platforms):
>         std::__basic_file<char>::__basic_file[in-charge](int*)
>         std::__basic_file<char>::__basic_file[not-in-charge](int*)
> 
> Appears --enable-thread=posix (on FreeBSD4)
>         std::__basic_file<char>::__basic_file[in-charge](pthread_mutex**)
>         std::__basic_file<char>::__basic_file[not-in-charge](pthread_mutex**)
> 
> Appears --enable-thread=posix (on GNU/linux)
>         std::__basic_file<char>::__basic_file[in-charge](pthread_mutex_t*)
>         std::__basic_file<char>::__basic_file[not-in-charge](pthread_mutex_t*)
> 
> It is unclear that code which references this ever goes into an app
> binary (i.e. it's use might really be contained within the library and
> it was just accidentally exported, I haven't checked any real cases yet).

it's explicitly exported in config/linker-map.gnu with the line:

GLIBCPP_3.2 {

  global:

    # Names inside the 'extern' block are demangled names.
    # All but the last are terminated with a semicolon.
    extern "C++"
    {
      std::[A-Za-z]*;
      std::__throw_*;
      std::__basic_file*;   <-----------

-benjamin