Fix stdatomic case in rbimpl_atomic_u64_fetch_add

This was failing on crossruby, likely because HAVE_GCC_ATOMIC_BUILTINS
was true, but HAVE_GCC_ATOMIC_BUILTINS_64 was false. We probably should
have feature detection of 64-bit stdatomics like we do for GCC, but for
now let's keep rbimpl_atomic_u64_fetch_add in sync with load/set.

Author: jhawthorn

ruby_atomic.h

@@ -43,6 +43,8 @@ rbimpl_atomic_u64_load_relaxed(const volatile rbimpl_atomic_uint64_t *value)
     uint64_t val = *value;
     return atomic_cas_64(value, val, val);
 #else
+    // TODO: stdatomic
+
     return *value;
 #endif
 }
@@ -58,6 +60,8 @@ rbimpl_atomic_u64_set_relaxed(volatile rbimpl_atomic_uint64_t *address, uint64_t
 #elif defined(__sun) && defined(HAVE_ATOMIC_H) && (defined(_LP64) || defined(_I32LPx))
     atomic_swap_64(address, value);
 #else
+    // TODO: stdatomic
+
     *address = value;
 #endif
 }
@@ -72,9 +76,9 @@ rbimpl_atomic_u64_fetch_add(volatile rbimpl_atomic_uint64_t *ptr, uint64_t val)
     return InterlockedExchangeAdd64((volatile LONG64 *)ptr, val);
 #elif defined(__sun) && defined(HAVE_ATOMIC_H) && (defined(_LP64) || defined(_I32LPx))
     return atomic_add_64_nv(ptr, val) - val;
-#elif defined(HAVE_STDATOMIC_H)
-    return atomic_fetch_add_explicit((_Atomic uint64_t *)ptr, val, memory_order_seq_cst);
 #else
+    // TODO: stdatomic
+
     // Fallback using mutex for platforms without 64-bit atomics
     static rb_native_mutex_t lock = RB_NATIVE_MUTEX_INITIALIZER;
     rb_native_mutex_lock(&lock);