re PR target/21760 (Powerpc atomic builtins missing PPC405 errata)

        PR target/21760
        * config/rs6000/rs6000.h (PPC405_ERRATUM77): New.
        * config/rs6000/rs6000.md: Move atomic instructions to ...
        * config/rs6000/sync.md: Here.
        Change sync_compare_and_swap<mode> to define_expand.  All stwcx
        patterns test PPC405_ERRATUM77.

From-SVN: r101268