unwinding fallbacks for powerpc32 on aix 5.2 and 5.3

[David Edelsohn]

libgcc/
        * config/rs6000/aix-unwind.h (ucontext_for): Helper for ...
        (ppc_aix_fallback_frame_state): New, implementation of ...
        (MD_FALLBACK_FRAME_STATE_FOR): Define.

Please do not create another definition of the register number for LR.
Earlier in the file it is defined as R_LR. Dropping in a completely
self-contained chunk with its own naming is confusing. LR_REGNO is
fine, but please define the necessary registers at the beginning of
the file and use them consistently throughout.

Okay with that cleanup.

Thanks, David