Properly implement lwarx and stwcx.

[benvanik]

They currently don't do the right thing w.r.t. memory. I'm not sure what I'm doing (acquire/release) does what I think it does, and it's slightly different than the PPC.

[chaoscode]

http://publib.boulder.ibm.com/infocenter/pseries/v5r3/index.jsp?topic=/com.ibm.aix.aixassem/doc/alangref/lwarx.htm

Per the examples lwarx is a load and reverse on the registry and stwcx copies it if it stays reversed.

• Assume that GPR 4 contains the new value to be stored.

• Assume that GPR 3 contains the address of the word

• to be loaded and replaced.
  loop:

   lwarx   r5,0,r3          # Load and reserve
  
  stwcx.  r4,0,r3          # Store new value if still
  
                           # reserved
  
  bne-    loop             # Loop if lost reservation
  

• The new value is now in storage.

• The old value is returned to GPR 4.