Add mutex initializer for wasm-c-api engine operations (#1656)

The host embedder may new/delete wasm-c-api engine simultaneously
in multiple threads, which requires lock for the operations. Since there
isn't one time called global init/destroy APIs provided by wasm-c-api,
we define a global lock and initialize it with thread mutex initializer if
the platform supports that, and use it to lock the operations of engine.

If the platform doesn't support thread mutex initializer, we require
developer to create the lock by himself to ensure the thread-safe of the
engine operations.

core/iwasm/include/wasm_c_api.h

@@ -145,6 +145,19 @@ WASM_API_EXTERN own wasm_config_t* wasm_config_new(void);
 
 WASM_DECLARE_OWN(engine)
 
+/**
+ * Create a new engine
+ *
+ * Note: for the engine new/delete operations, including this,
+ * wasm_engine_new_with_config, wasm_engine_new_with_args, and
+ * wasm_engine_delete, if the platform has mutex initializer,
+ * then they are thread-safe: we use a global lock to lock the
+ * operations of the engine. Otherwise they are not thread-safe:
+ * when there are engine new/delete operations happening
+ * simultaneously in multiple threads, developer must create
+ * the lock by himself, and add the lock when calling these
+ * functions.
+ */
 WASM_API_EXTERN own wasm_engine_t* wasm_engine_new(void);
 WASM_API_EXTERN own wasm_engine_t* wasm_engine_new_with_config(own wasm_config_t*);