MDEV-5766 - my_atomic_load does memory writes

my_atomic_load() is implemented as __sync_fetch_and_or(var, 0) which
writes or-ed value back to var. Memory writes as such have worse
performance and scalability than reads.

gcc 4.7 and up offers better facility for atomic loads/stores. Use it
whenever it is available.

include/atomic/gcc_builtins.h

@@ -31,6 +31,12 @@
 #define make_atomic_store_body(S)  *a= v
 #define MY_ATOMIC_MODE "gcc-builtins-up"
 
+#elif defined(__ATOMIC_SEQ_CST)
+#define MY_ATOMIC_MODE "gcc-builtins-smp"
+#define make_atomic_load_body(S)                    \
+  ret= __atomic_load_n(a, __ATOMIC_SEQ_CST)
+#define make_atomic_store_body(S)                   \
+  __atomic_store_n(a, v, __ATOMIC_SEQ_CST)
 #else
 #define MY_ATOMIC_MODE "gcc-builtins-smp"
 #define make_atomic_load_body(S)                    \