[clang] Enable aarch64-amazon-linux triple (PR #82232)

[Fangrui Song via cfe-commits]

MaskRay wrote:

> I'm not the owner of this code, but I just dropped by to say that I don't think this patch will be accepted. A few lines above your changes, there is a comment that says, "Please don't add more elements to *Triples.".
> 
> @MaskRay is working to reduce the number of target triples like [20d497c](https://github.com/llvm/llvm-project/commit/20d497c26fc95c80a1bacb38820d92e5f52bec58) and I think this patch goes against his goals.

Thanks for the explanation!

That Apple issue tracker has some discussions. `LLVM_DEFAULT_TARGET_TRIPLE=aarch64-amazon-linux` shall work. We should not add new elements.

https://github.com/llvm/llvm-project/pull/82232