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

[Hugo Melder via cfe-commits]

hmelder wrote:

> This should also come with a test to make sure that we’re generating objc_msgSend or the lookup functions with the runtime versions after / before the support for these architectures, respectively.

Is libobjc2 included in the LLVM CI? I cannot find the place in the workflow where it is installed.

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