Add one-off gcc release version build workflow (Ubuntu 24.04 migration)

[mattgodbolt-molty]

(I'm Molty, an AI assistant acting on behalf of @mattgodbolt)

Adds build-gcc-release.yml — a workflow_dispatch workflow for building a specific gcc release version on demand.

Needed for the Ubuntu 24.04 migration: after merging the gcc-builder PR to remove old binutils version pins, we need to rebuild ~29 old gcc versions (5.x–11.3) with binutils 2.44.

Usage

# Trigger a single build
gh workflow run build-gcc-release.yml -R compiler-explorer/compiler-workflows -f version=5.5.0

# Monitor
gh run list -R compiler-explorer/compiler-workflows --workflow=build-gcc-release.yml --limit 5

# Batch (all 29 affected versions)
for ver in 5.1.0 5.2.0 5.3.0 5.4.0 5.5.0 \
           6.1.0 6.2.0 6.3.0 6.4.0 \
           7.1.0 7.2.0 7.3.0 7.4.0 7.5.0 \
           8.1.0 8.2.0 8.3.0 8.4.0 8.5.0 \
           9.1.0 9.2.0 9.3.0 9.4.0 \
           10.1.0 10.2.0 10.3.0 \
           11.1.0 11.2.0 11.3.0; do
  gh workflow run build-gcc-release.yml -R compiler-explorer/compiler-workflows -f version=$ver
done

Each build uploads to s3://compiler-explorer/opt/gcc-{VERSION}.tar.xz. Old release versions have no existing .buildrevs entry so the revision check won't skip them.

[mattgodbolt-molty]

Closing — bespoke-build.yaml in the infra repo already handles this. No new workflow needed in compiler-workflows.