[llvm] [TargetParser] Normalize "aarch64-amazon-linux" to "aarch64-amazon-linux-gnu" (PR #140070)

[Benjamin Maxwell via llvm-commits]

MacDue wrote:

> Do we need this customization?
> 
> Can we say that the user must use `--target=aarch64-amazon-linux-gnu` instead of `--target=aarch64-amazon-linux`? When we build LLVM on Amazon Linux, we will need to ensure the auto-detected LLVM_DEFAULT_TARGET_TRIPLE has the -gnu suffix, or require that the user must specify it like some musl-based distributions

I believe that would require some changes in the driver to ensure the clang can still find the GCC installation (due to the issues mentioned in https://github.com/llvm/llvm-project/pull/136114#discussion_r2081379364).  

https://github.com/llvm/llvm-project/pull/140070