rs6000.h (RETURN_ADDR_RTX): Add definition to support __builtin_return_address and __builtin_frame_address...

        * rs6000.h/rs6000.h (RETURN_ADDR_RTX): Add definition to support
        __builtin_return_address and __builtin_frame_address on the
        rs6000.  This is necessary to support exception handling in g++ on
        the rs6000.
Fixes eh6.C on rs6000.

From-SVN: r9054