arch-power: Fix fixed-point load and store instructions

arch-power: Fix fixed-point load and store instructions

This fixes the following load and store instructions as a result
of the change in register widths:
  * Load Word and Zero (lwz)
  * Load Word and Zero Indexed (lwzx)
  * Load Word and Zero with Update (lwzu)
  * Load Word and Zero with Update Indexed (lwzux)
  * Load Word Algebraic (lwa)
  * Load Word And Reserve Indexed (lwarx)
  * Store Word (stw)
  * Store Word Indexed (stwx)
  * Store Word with Update (stwu)
  * Store Word with Update Indexed (stwux)
  * Store Word Conditional Indexed (stwcx.)

This also fixes disassembly generation for all of the above.

Change-Id: I1a25cdb5ffe86145b7ffcf2c2bd7b27048a415d2
Signed-off-by: Sandipan Das <sandipan@linux.vnet.ibm.com>