Port GDB to powerpc-lynx178.

Port GDB to powerpc-lynx178.

gdb/ChangeLog:

        * defs.h (enum gdb_osabi): Add GDB_OSABI_LYNXOS178.
        * osabi.c (gdb_osabi_names): Add entry for GDB_OSABI_LYNXOS178.
        * xcoffread.c (xcoff_get_core_n_import_files): New function.
        (xcoff_get_n_import_files): New function.
        * xcoffread.h (xcoffread.h): Add declaration.
        * rs6000-aix-tdep.c: #include "xcoffread.h".
        (rs6000_aix_osabi_sniffer): Do not return GDB_OSABI_AIX for
        XCOFF executables that do not depend on any shared library.
        * rs6000-lynx178-tdep.c: New file.
        * configure.tgt: Add powerpc-*-lynx*178 handling.
        * Makefile.in (ALL_TARGET_OBS): Add rs6000-lynx178-tdep.o.
        (ALLDEPFILES): Add rs6000-lynx178-tdep.c.