powerpc: Emulate sync instruction variants

Reserved fields of the sync instruction have been used for other
instructions (e.g. lwsync).  On processors that do not support variants
of the sync instruction, emulate it by executing a sync to subsume the
effect of the intended instruction.

Signed-off-by: James Yang <James.Yang@freescale.com>
[scottwood@freescale.com: whitespace and subject line fix]
Signed-off-by: Scott Wood <scottwood@freescale.com>