Enhance XIP and add XIP document (#863)

Auto detect whether file is XIP file before loading module in posix like and
linux-sgx platforms, and if yes, mmap executable memory automatically to
run the XIP file.
Add document about XIP feature.
Enable test spec cases with XIP feature.

core/iwasm/include/wasm_export.h

@@ -235,6 +235,17 @@ wasm_runtime_free(void *ptr);
 WASM_RUNTIME_API_EXTERN package_type_t
 get_package_type(const uint8_t *buf, uint32_t size);
 
+/**
+ * Check whether a file is an AOT XIP (Execution In Place) file
+ *
+ * @param buf the package buffer
+ * @param size the package buffer size
+ *
+ * @return true if success, false otherwise
+ */
+WASM_RUNTIME_API_EXTERN bool
+wasm_runtime_is_xip_file(const uint8_t *buf, uint32_t size);
+
 /**
  * It is a callback for WAMR providing by embedding to load a module file
  * into a buffer