Properly use ATOMIC_*_LOCK_FREE preprocessor constants

The ATOMIC_*_LOCK_FREE can evalutate either 0, 1, or 2 which indicate the
lock-free property of the corresponding atomic types (both signed and unsigned).

	Value	Explanation
	-----	--------------------------------------
	  0	The atomic type is never lock-free
	  1	The atomic type is sometimes lock-free
	  2	The atomic type is always lock-free
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