kleidiai: Rework KleidiAI Build System/Integration

[JonathanC-ARM]

Overview

This PR reworks how kleidiai is included into llama.cpp via cmake.
The intent here is to remove the requirement for manually adding kernel source files, -march flags and other things that are specific to the kleidiai library and instead relying on the libraries own CMakeLists.

Various ifdef guards are also removed from ggml/src/ggml-cpu/kleidiai/kernels.cpp which is safe to do as kernel selection will be runtime feature safe from the changes included in PR 1 - #26076

Additional information

This pr is intended to be 2/3 in a series of pr's to improve the KleidiAI with its cpu feature detection, build system and documentation.

PR 1 - #26076
PR 3 - #26078

Requirements

• I have read and agree with the contributing guidelines - YES
• AI usage disclosure: YES - Ai was used to help understand llama.cpp

[ggml-gh-bot]

Hi @JonathanC-ARM, thanks for your contribution!

Per our contribution guidelines, the automated PR checker found the following issue(s) that need your attention:

• Multiple open PRs from a new contributor: We limit new contributors (those without a previously merged PR) to 1 open PR at a time. You currently have 2 open PRs.

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Please note that maintainers reserve the right to make final decisions on PRs. If you believe there is a mistake, please comment below.

[JonathanC-ARM]

Hi @JonathanC-ARM, thanks for your contribution!

Per our contribution guidelines, the automated PR checker found the following issue(s) that need your attention:

* **Multiple open PRs from a new contributor**: We limit new contributors (those without a previously merged PR) to **1 open PR at a time**. You currently have 2 open PRs.

Please note that maintainers reserve the right to make final decisions on PRs. If you believe there is a mistake, please comment below.

This is correct, however the approach here from my part was to give context to a series of work. This pr is draft and is not required to be acted upon and will only be updated and pursued if and when PR 1 in this series is accepted.

[JonathanC-ARM]

@chaxu01 @taronaeo would it be possible to get a review on this?

[taronaeo]

Lemme look into it the day after tomorrow :)