fix broken constraints for powerpc atomic cas asm

fix broken constraints for powerpc atomic cas asm

the register constraint for the address to be accessed did not convey
that the asm can access the pointed-to object. as far as the compiler
could tell, the result of the asm was just a pure function of the
address and the values passed in, and thus the asm could be hoisted
out of loops or omitted entirely if the result was not used.