To match reality and allow using these functions with pointers on const
objects.
Remove the '+' modifier in the atomic_load_acq_64_i586()'s inline asm
statement's constraint for '*p' (the value to load). CMPXCHG8B always
writes back some value, even when the value exchange does not happen, in
which case what was read is written back. atomic_load_acq_64_i586()
further takes care of the operation atomically writing back the same
value that was read in any case. All in all, this makes the inline
asm's write back totally undetectable by any other code, whether
executing on other CPUs or code on the same CPU before and after the
call to atomic_load_acq_64_i586().