JDK-8286825 made some renaming of hotspot file and method, the following renaming was missing in riscv:
- universalNativeInvoker*--> downcallLinker*
- 'native invoker' -> 'downcall stub'

build errors:

```
/home/jiangfeilong/workspace/jdk/src/hotspot/cpu/riscv/downcallLinker_riscv.cpp:27:10: fatal error: prims/universalNativeInvoker.hpp: No such file or directory
 #include "prims/universalNativeInvoker.hpp"
          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```

```
/home/jiangfeilong/workspace/jdk/src/hotspot/cpu/riscv/downcallLinker_riscv.cpp:36:74: error: no 'RuntimeStub* DowncallLinker::make_native_invoker(BasicType*, int, BasicType, const ABIDescriptor&, const GrowableArray<VMRegImpl*>&, const GrowableArray<VMRegImpl*>&, bool)' member function declared in class 'DowncallLinker'
                                                  bool needs_return_buffer) {
                                                                          ^
cc1plus: error: unrecognized command line option '-Wno-cast-function-type' [-Werror]
cc1plus: all warnings being treated as errors
```