Publish a Linux ARM64 flatc in releases

### Request

Please publish a Linux arm64/aarch64 `flatc` binary alongside the existing release assets.

### Current state

The latest release (`v25.12.19-2026-02-06-03fffb2`) ships: Linux g++, Linux Clang, Mac, Mac Intel and Win builds.

Both Linux archives are built on x86_64 runners, so macOS is the only platform with an arm64 binary. Linux arm64 users have to build `flatc` from source or rely on a distro package, which makes it hard to pin the exact compiler version that generated a checked-in schema.

### Why it matters

Linux arm64 is no longer niche: CI runners (GitHub's own `ubuntu-24.04-arm`), [Graviton](https://en.wikipedia.org/wiki/AWS_Graviton) build machines, and arm64 containers on Apple silicon dev machines are all common. Build scripts that provision `flatc` by downloading a release asset have to special-case Linux arm64 with no good answer; picking a Linux asset by OS alone silently downloads an x86_64 binary that fails at exec time.

Version-matching is the real pain point: generated code is typically validated against a specific `flatc` version, and a distro package or a from-source build is hard to hold at the exact release the rest of the toolchain is pinned to.

### Prior

- #7694 asked for this in 2022.
- #8583 was a PR that added exactly this, building `flatc` for `aarch64` on the `ubuntu-24.04-arm` runner. It was never reviewed and the stale bot auto-closed it in December 2025.

Since GitHub-hosted arm64 runners are now free for public repos, the release workflow change looks small — essentially the #8583 diff.

Review the release workflow and the prior #8583 change first, focusing on how the existing Linux g++ and Clang assets are built. Add a Linux arm64/aarch64 flatc asset using the ubuntu-24.04-arm runner, then verify that the release includes it alongside the existing platform builds.

Written by the indexing model from the issue text.