[clang] [Clang][Driver][AArch64] Add support for aarch64-amazon-linux triple (PR #109263)

[Peter Waller via cfe-commits]

peterwaller-arm wrote:

@MaskRay please would you kindly reconsider aarch64-amazon-linux triple as an exception given the presence of x86_64 equivalent?

I've explored that `LLVM_DEFAULT_TARGET_TRIPLE` if unspecified comes from `LLVM_INFERRED_HOST_TRIPLE` which comes from config.guess in here: https://github.com/llvm/llvm-project/blob/592adb0b24aca2ccc88d9f5f1c427361897d1172/llvm/cmake/modules/GetHostTriple.cmake#L49

config.guess is copy of an external project with a different license; this is what reports `aarch64-unknown-linux-gnu`. For the time being it would seem fair to put aarch64 on the same footing as x86_64 here.

https://github.com/llvm/llvm-project/pull/109263