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

[Hugo Melder via cfe-commits]

hmelder wrote:

I assume we can split the test into two parts:
1. Checking if the driver correctly sets `-fobjc-dispatch-method`
2. Checking if frontend emits `objc_msgSend`

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