Replace atomicExch with __atomic_store_n (#818)

* Replace atomicExch with __atomic_store_n

* Remove extra semicolon

[ROCm/rccl commit: 4d20b4b758]

projects/rccl/src/collectives/device/common.h

@@ -22,7 +22,7 @@
 
 #ifdef __GFX9__
 #define STORE(DST, SRC) \
-  { __threadfence(); atomicExch((unsigned long long *)(DST), (SRC)); }
+  { __threadfence(); __atomic_store_n((DST), (SRC), __ATOMIC_RELAXED); }
 #else
 #define STORE(DST, SRC) \
   { __atomic_store_n((DST), (SRC), __ATOMIC_SEQ_CST); }