Target specific Relobj?

[Doug Kwan (關振德)]

Hi Ian,

    I need to attach some ARM specific information to Relobj objects.
To do that I can think of two ways

    1.  Use an Unordered_map<Relobj*, Arm_specific_info*>
    2.  Sub-classing Relobj (or Sized_relobj)

I actually prefer 2.  I would like to check make_elf_sized_object() to
check if a target has its own make_elf_sized_object.  If so, it calls
the target's own version.  We'll need to pass size and big_endian as
two extra parameters.  What do you think?

-Doug