[llvm] [NFC][AArch64] Organise extensions by archtecture version (PR #95898)

[Lucas Duarte Prates via llvm-commits]

pratlucas wrote:

> LGTM. At the risk of creating more churn, would it make sense to actually split them into separate files, one per architecture version? We already keep our downstream features in a separate file and it keeps things tidy. A second opinion from someone outside of Arm would be good.

I'd also like to get the opinion of someone from outside of Arm on this. However, I'd prefer to implement such a change in a separate PR to avoid the churn it would cause to the changes in #95805.

https://github.com/llvm/llvm-project/pull/95898