Add GCC runtime install action (12/24)

[MikeMcQuaid]

Eight GCC formulae share the same Linux runtime-link and specs
generation algorithm after installation.

• probe the active GCC and glibc runtime locations
• generate family-appropriate library, linker and header search paths
• keep the repeated algorithm behind one literal formula step

AI disclosure: using OpenAI Codex 5.6 Sol max with local review and
testing.

[Copilot]

Pull request overview

This PR adds a dedicated configure_gcc_runtime install-steps DSL action so GCC-family formulae on Linux can share a single post-install “runtime links + specs generation” algorithm, rather than duplicating the logic across multiple formulae.

Changes:

• Add a new configure_gcc_runtime DSL step and runner dispatch hook in Homebrew::InstallSteps.
• Implement the Linux GCC runtime/specs configuration logic as a Homebrew::InstallSteps::Runner private action.
• Update RuboCop install-step allowlists, add a runner dispatch spec, and document the new action in the Formula Cookbook.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
Library/Homebrew/test/rubocops/install_steps_spec.rb	Updates the allowed-step list expectations to include configure_gcc_runtime.
Library/Homebrew/test/install_steps_spec.rb	Adds a spec asserting the runner dispatches the new step.
Library/Homebrew/rubocops/shared/install_steps_helper.rb	Allows configure_gcc_runtime in steps blocks.
Library/Homebrew/install_steps/formula_actions.rb	Implements the GCC runtime configuration runner action (Linux-only).
Library/Homebrew/install_steps.rb	Adds the DSL method, runner dispatch case, and requires the new action file.
docs/Formula-Cookbook.md	Documents the new “repeated formula action” and when to use it.

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