[clang] ObjcRuntime.h: Add mips64, aarch64, and riscv64 to non-legacy dispatch (PR #76694)

[Hugo Melder via cfe-commits]

hmelder wrote:

Definition of -fobjc-dispatch-method:

```
def fobjc_dispatch_method_EQ : Joined<["-"], "fobjc-dispatch-method=">,
  HelpText<"Objective-C dispatch method to use">,
  Values<"legacy,non-legacy,mixed">,
  NormalizedValuesScope<"CodeGenOptions">, NormalizedValues<["Legacy", "NonLegacy", "Mixed"]>,
  MarshallingInfoEnum<CodeGenOpts<"ObjCDispatchMethod">, "Legacy">;
```

Which is used in CGObjCGNU::GenerateMessageSend

https://github.com/llvm/llvm-project/pull/76694