re PR libstdc++/51181 (libstdc++.so __sync_sub_and_fetch_4 linkage error causing many test suite failures on m68k-linux)

PR libstdc++/51181
        * libsupc++/eh_tm.cc (free_any_cxa_exception): Protect the use
        of __sync_sub_and_fetch with _GLIBCXX_ATOMIC_BUILTINS_4.

From-SVN: r181465