Don't call os_thread_get_stack_boundary unless we actually use it (#4264)

Previously, if the user sets their own stack boundary, we still compute
the thread stack boundary (which is expensive), then immediately discard
the result. This change makes the expensive call only if we need it for
sure.

Author: sjamesr

core/iwasm/common/wasm_exec_env.c

@@ -276,8 +276,6 @@ wasm_exec_env_restore_module_inst(
 void
 wasm_exec_env_set_thread_info(WASMExecEnv *exec_env)
 {
-    uint8 *stack_boundary = os_thread_get_stack_boundary();
-
 #if WASM_ENABLE_THREAD_MGR != 0
     os_mutex_lock(&exec_env->wait_lock);
 #endif
@@ -286,9 +284,11 @@ wasm_exec_env_set_thread_info(WASMExecEnv *exec_env)
         /* WASM_STACK_GUARD_SIZE isn't added for flexibility to developer,
            he must ensure that enough guard bytes are kept. */
         exec_env->native_stack_boundary = exec_env->user_native_stack_boundary;
-    else
+    else {
+        uint8 *stack_boundary = os_thread_get_stack_boundary();
         exec_env->native_stack_boundary =
             stack_boundary ? stack_boundary + WASM_STACK_GUARD_SIZE : NULL;
+    }
     exec_env->native_stack_top_min = (void *)UINTPTR_MAX;
 #if WASM_ENABLE_THREAD_MGR != 0
     os_mutex_unlock(&exec_env->wait_lock);