hexagon: add alternative Hexagon NPU backend implementation

[zhouwg-jeffzhou]

Overview

Follow‑up to #26373

This PR adds FastRPC-based ggml-hexagon, an alternative backend for Qualcomm Hexagon NPU on Android, which can coexist with the existing official dspqueue-based ggml-hexagon implementation.

Additional information

details can be found at RFC-26227

Requirements

• I have read and agree with the contributing guidelines
• AI usage disclosure: YES - used AI assistance for implementation help. I reviewed every changed line, tested locally, and take full responsibility for this PR.

[ggml-gh-bot]

Hi @zhouwg-jeffzhou, 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.

• Large PR: Large changes require prior discussion (e.g. an issue or RFC) and maintainers may not be able to review this PR as-is. Consider splitting it into smaller, focused 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.

[zhouwg-jeffzhou]

@ggml-gh-bot, thanks for your kind reminder. I have already converted my opening PR to drat.

[zhouwg-jeffzhou]

@max-krasnyansky, sorry to bother you. I have carefully studied your feedback in #26373, thank you very much for providing many valuable suggestions. This new PR addresses your suggestions, and I would like to make some solid contributions to ggml‑hexagon.

[zhouwg-jeffzhou]

after submitting the PR, I spent several hours reviewing source files and scripts on the GitHub page, trying to strictly align with Qualcomm's original behavior.

It sees that ggml/src/ggml‑hexagon/CMakeLists.txt and ggml/src/ggml‑hexagon/htp/CMakeLists.txt are already aligned with upstream.

I am not sure whether I should modify the backend name from Hexagon‑cDSP to HTP, so that the backend name is also strictly aligned with Qualcomm's original behavior.