x86, cmpxchg: Move 64-bit set64_bit() to match 32-bit

Reduce arbitrary differences between 32 and 64 bits.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Link: http://lkml.kernel.org/r/4E5BCC40.3030501@goop.org
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>