gh-133171: Re-enable JUMP_BACKWARD to free-threading build

Author: corona10

Python/bytecodes.c

@@ -2940,9 +2940,10 @@ dummy_func(
         };
 
         tier1 op(_SPECIALIZE_JUMP_BACKWARD, (--)) {
-        #if ENABLE_SPECIALIZATION
+        #if ENABLE_SPECIALIZATION_FT
             if (this_instr->op.code == JUMP_BACKWARD) {
-                this_instr->op.code = tstate->interp->jit ? JUMP_BACKWARD_JIT : JUMP_BACKWARD_NO_JIT;
+                uint8_t desired = tstate->interp->jit ? JUMP_BACKWARD_JIT : JUMP_BACKWARD_NO_JIT;
+                FT_ATOMIC_STORE_UINT8_RELAXED(this_instr->op.code, desired);
                 // Need to re-dispatch so the warmup counter isn't off by one:
                 next_instr = this_instr;
                 DISPATCH_SAME_OPARG();