flambda-backend: Remove C11 _Atomic typedefs (#1994)

remove _Atomic typedefs

Author: TheNumbat

runtime4/caml/camlatomic.h

@@ -42,8 +42,10 @@ using std::memory_order_seq_cst;
 
 #include <stdatomic.h>
 #define ATOMIC_UINTNAT_INIT(x) (x)
-typedef _Atomic uintnat atomic_uintnat;
-typedef _Atomic intnat atomic_intnat;
+
+// CR ocaml 5 runtime: provide these typedefs (requires C11)
+// typedef _Atomic uintnat atomic_uintnat;
+// typedef _Atomic intnat atomic_intnat;
 
 #elif defined(__GNUC__)