Enable ref types and bulk memory by default for wamrc (#838)

Enable ref types feature and bulk memory feature by default for wamrc
and provide "--disable-ref-types", "--disable-bulk-memory"  to disable
them.
And remove the ref_type_flag option in wasm_loader.c which is used to
control whether to enable ref types or not when ENABLE_REF_TYPES
macro is enabled in wamrc. As the wasm binary format with ref types
is compatible with the binary format before, we can remove the option.
Also update the spec test scripts.

samples/wasm-c-api/CMakeLists.txt

@@ -157,7 +157,7 @@ foreach(EX ${EXAMPLES})
   # generate .aot file
   if(${WAMR_BUILD_AOT} EQUAL 1)
     add_custom_target(${EX}_AOT
-      COMMAND ${WAMRC} --enable-ref-types -o ${PROJECT_BINARY_DIR}/${EX}.aot
+      COMMAND ${WAMRC} -o ${PROJECT_BINARY_DIR}/${EX}.aot
         ${PROJECT_BINARY_DIR}/${EX}.wasm
       DEPENDS ${EX}_WASM
       BYPRODUCTS ${PROJECT_BINARY_DIR}/${EX}.aot