Automate LLVM_COMMIT update on upstream release

[ppenna]

Problem

The LLVM_COMMIT ARG in the Dockerfile must be manually updated whenever a new llvm-project commit is needed. While the repository_dispatch path (llvm-stage1-release) overrides this at build time via --build-arg, the default value in the Dockerfile goes stale — breaking manual builds, push-to-main triggers, and workflow_dispatch runs.

Proposed Solution

Add a workflow (or extend nanvix-release.yml in llvm-project) that automatically opens a PR to bump LLVM_COMMIT in this repo's Dockerfile after each successful llvm-project release. Options:

1. Dependabot-style workflow in this repo: triggered by repository_dispatch from llvm-project, runs sed to update the commit SHA, commits, and opens a PR.
2. Dispatch payload auto-commit: on llvm-stage1-release dispatch, a pre-build step updates the Dockerfile default and pushes directly to main (simpler but less reviewable).
3. Renovate/Dependabot custom datasource for git commit pinning.

Context

• Current manual update: PR Update LLVM_COMMIT to include direct linker fix #3
• Release chain: binutils → llvm stage 0 → newlib → llvm stage 1 → toolchain-clang
• Same pattern likely needed for BINUTILS_COMMIT and NEWLIB_COMMIT args