Fix a typo that was causing several validation tests to hang.
(Doing cmpxchg8b (%eax) isn't going to work very well.) I am
wondering if something is wrong with the general implementation
of ck_pr_bts_64 and ck_pr_btc_64 because it's pretty clear that
with the stack tests passing, ck_pr_cas_32_2_value works fine.