[Qemu-ppc] Implement instructions: yield, mdoio, mdoom

[alarson]

In target-ppc/translate.c, the gen_or() function now has the ability
to pause the CPU via gen_pause().  However gen_or() does not implement
the PPC instructions yield (rs,ra,rb=27), mdoio (29), or mdoom (30),
which could be handled by simply calling gen_pause() for them.

I could either:
  1. Add explicit code for registers 27, 29, and 30, or
  2. Change the existing code to call gen_pause() whenever rs!=0
    (simple noop).

My rationale for #2 is that it is simpler, and seems likely to be
right for future encodings of "or".

Comments solicited.

Aaron.