Export each of the memory_order enumerators

The lib/atomic.h exports memory_order enum into the global namespace,
but none of its enumerators. According to Section 7.3.3 "The using
declaration" of the C++ standard: "specifying an enumeration name
in a using-declaration does not declare its enumerators in
the using-declaration's declarative region." See also:
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0943r1.html

Signed-off-by: Sergei Korneichuk <[email protected]>

lib/atomic.h

@@ -26,6 +26,12 @@
 # include <atomic>
 using std::atomic_flag;
 using std::memory_order;
+using std::memory_order_relaxed;
+using std::memory_order_consume;
+using std::memory_order_acquire;
+using std::memory_order_release;
+using std::memory_order_acq_rel;
+using std::memory_order_seq_cst;
 
 using std::atomic_bool;
 using std::atomic_char;