RISC-V: GCC 11 warning in c1_CodeStubs.hpp

Current GCC 11 RISC-V builds fail with:

```
* For target hotspot_variant-server_libjvm_objs_c1_LIRAssembler.o:
In file included from /home/buildbot/worker/build-jdkX-debian12/build/src/hotspot/share/c1/c1_LIRAssembler.hpp:28,
                 from /home/buildbot/worker/build-jdkX-debian12/build/src/hotspot/share/c1/c1_LIRAssembler.cpp:30:
In member function 'void PatchingStub::install(MacroAssembler*, LIR_PatchCode, Register, CodeEmitInfo*)',
    inlined from 'void PatchingStub::install(MacroAssembler*, LIR_PatchCode, Register, CodeEmitInfo*)' at /home/buildbot/worker/build-jdkX-debian12/build/src/hotspot/share/c1/c1_CodeStubs.hpp:423:8,
    inlined from 'void LIR_Assembler::patching_epilog(PatchingStub*, LIR_PatchCode, Register, CodeEmitInfo*)' at /home/buildbot/worker/build-jdkX-debian12/build/src/hotspot/share/c1/c1_LIRAssembler.cpp:46:17:
/home/buildbot/worker/build-jdkX-debian12/build/src/hotspot/share/c1/c1_CodeStubs.hpp:440:25: error: 'this' pointer is null [-Werror=nonnull]
  440 | n_move->set_offset(field_offset);
      | ~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~
In file included from /home/buildbot/worker/build-jdkX-debian12/build/src/hotspot/share/code/nativeInst.hpp:30,
                 from /home/buildbot/worker/build-jdkX-debian12/build/src/hotspot/share/c1/c1_CodeStubs.hpp:33,
                 from /home/buildbot/worker/build-jdkX-debian12/build/src/hotspot/share/c1/c1_LIRAssembler.hpp:28,
                 from /home/buildbot/worker/build-jdkX-debian12/build/src/hotspot/share/c1/c1_LIRAssembler.cpp:30:
/home/buildbot/worker/build-jdkX-debian12/build/src/hotspot/cpu/riscv/nativeInst_riscv.hpp: In member function 'void LIR_Assembler::patching_epilog(PatchingStub*, LIR_PatchCode, Register, CodeEmitInfo*)':
/home/buildbot/worker/build-jdkX-debian12/build/src/hotspot/cpu/riscv/nativeInst_riscv.hpp:420:8: note: in a call to non-static member function 'void NativeMovRegMem::set_offset(int)'
  420 | void set_offset(int x);
   ... (rest of output omitted)
```

I think the compiler is smart/dumb enough to see through `Unimplemented()` here and then complain about the nullity:

```
inline NativeMovRegMem* nativeMovRegMem_at (address addr) {
  Unimplemented();
  return NULL;
}
```