thread mgr: Free aux stack only when it was allocated (#3282)

When thread manager is enabled, the aux stack of exec_env may be allocated
by wasm_cluster_allocate_aux_stack or disabled by setting aux_stack_bottom
as UINTPTR_MAX directly. For the latter, no need to free it.

And fix an issue when paring `--gc-heap-size=n` argument for iwasm, and
fix a variable shadowed warning in fast-jit.

core/iwasm/common/wasm_exec_env.h

@@ -117,6 +117,9 @@ typedef struct WASMExecEnv {
 
     /* whether current thread is detached */
     bool thread_is_detached;
+
+    /* whether the aux stack is allocated */
+    bool is_aux_stack_allocated;
 #endif
 
 #if WASM_ENABLE_GC != 0