Put in place the framework necessary for multiarching the hppa targets.

   Put in place the framework necessary for multiarching the hppa targets.
       * hppa-tdep.c (hppa_gdbarch_init): New function.
       (hppa_dump_tdep): New function.
       (_initialize_hppa_tdep): Register the hppa gdbarch init function and
       tdep structure dumper.
       * config/pa/tm-hppa.h (GDB_MULTI_ARCH): New macro, defined to 0
       until the multi-arching conversion has partially been completed.