Minor dubious fix for am33 gas

[Jeffrey A Law]

  In message <orr9cqj52y.fsf@zecarneiro.lsd.ic.unicamp.br>you write:
  > --=-=-=
  > 
  > I don't understand why the ``special'' registers introduced in AM33
  > are not listed in `other_registers'.  Does anybody?  I think it would
  > be beneficial to get saner error messages, but it might break AM30
  > programs that inadvertently used the register names as symbol names.
  > Is that the reason?  If not, is this ok to install?
Oversight most likely.  Though as you note, recognizing them may cause
problems with older am30/mn10300 code if they used those names for in
assembly code.

Probably the thing to do is only recognize them as other registers when
in am33 mode.  I don't think the existing *_register_name interface has
that capability, so you'll probably need a new function 
am33_other_register_name and an appropriate code block to use it based
on the value of current_machine.

jeff