Implement native function pointer check, addr conversion and register, update documents (#185)

Modified WASM runtime API:
- wasm_runtime_module_malloc()
- wasm_runtime_lookup_function()
Introduced runtime API
- wasm_runtime_register_natives()

product-mini/platforms/linux-sgx/CMakeLists.txt

@@ -113,4 +113,3 @@ add_library (vmlib
              ${IWASM_AOT_SOURCE}
              ${IWASM_COMPL_SOURCE})
 
-add_library (extlib ext_lib_export.c)