target-ppc: gen_pause for instructions: yield, mdoio, mdoom, miso

target-ppc: gen_pause for instructions: yield, mdoio, mdoom, miso

Call gen_pause for all "or rx,rx,rx" encodings other nop.  This
provides a reasonable implementation for yield, and a better
approximation for mdoio, mdoom, and miso.  The choice to pause for all
encodings !=0 leverages the PowerISA admonition that the reserved
encodings might change program priority, providing a slight "future
proofing".

Signed-off-by: Aaron Larson <[email protected]>
Acked-by: Benjamin Herrenschmidt <[email protected]>
Signed-off-by: David Gibson <[email protected]>