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

[Benjamin Maxwell via llvm-commits]

MacDue wrote:

> Should we be consistent and also add `x86_64-amazon-linux-gnu` -> `x86_64-amazon-linux

Note: This PR does normalizes things the other way around `x86_64-amazon-linux` -> `x86_64-amazon-linux-gnu` (the knowing Amazon Linux has a GNU environment is needed for some optimizations). The x86 case is already supported (added a test to show that), as this normalization is just looking for Amazon Linux. 

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