Document set/show debug aix-solib.

gdb/doc/ChangeLog:

        * gdb.texinfo (Debugging Output): Add documentation of
        "set debug aix-solib" and "show debug aix-solib" commands.
[doco] Document the XML format used for AIX shared lib support.

gdb/doc/ChangeLog:

        * gdbint.texinfo (Native Debugging): Add "AIX Shared Library
        Support" subsection documenting the XML format used to transfer
        shared library info on AIX.