Select the native architecture's dynamic linker on Linux

[p-linnane]

On ARM64 Linux with amd64 multi-arch libc installed, OS::Linux::Ld.system_ld_so returned the first executable path from a flat, cross-architecture list, so $HOMEBREW_PREFIX/lib/ld.so was symlinked to /lib64/ld-linux-x86-64.so.2 and ARM binaries stopped running. Reported in discussion #6179.

DYNAMIC_LINKERS is now keyed by Hardware::CPU.arch and only the native architecture's candidates are searched, so a foreign loader can no longer be selected when the native one is missing.

The second commit fixes something that surfaced while testing the first. brew tests --changed could select a spec that OS filtering then removed, leaving RSpec invoked with an empty file list and failing brew lgtm on macOS. --changed now warns and returns, while every other invocation fails, including an explicit --only for a spec the current OS filters out, rather than exiting green without running anything.

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• I did not use AI/LLM to create this PR, or I disclosed the tool/model below and reviewed its output; I did not attribute commits to AI and will answer maintainer questions and review comments myself without AI/LLM.

Claude Code (Opus 5) drafted the implementation and tests; I reviewed the diff, verified the new tests fail without the fix and pass with it, and ran brew lgtm --online plus targeted specs.

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[Copilot]

Pull request overview

This PR fixes Linux dynamic linker selection so Homebrew doesn’t accidentally choose a foreign-architecture loader (e.g., x86_64) on multi-arch systems where the native loader (e.g., arm64) is present, preventing broken execution of native binaries. It also hardens brew tests so OS-specific filtering can’t lead to “successful” runs where no specs actually execute.

Changes:

• Key OS::Linux::Ld::DYNAMIC_LINKERS by Hardware::CPU.arch and search only native-arch candidates in system_ld_so.
• Add/adjust Linux Ld specs to cover native-vs-foreign loader preference.
• Update brew tests to warn+return for --changed when OS filtering removes all tests, and to error for explicit invocations that result in no runnable tests; add corresponding unit tests.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
Library/Homebrew/os/linux/ld.rb	Restricts dynamic linker discovery to native-architecture candidates.
Library/Homebrew/test/os/linux/ld_spec.rb	Updates/extends specs to validate native linker preference behavior.
Library/Homebrew/dev-cmd/tests.rb	Adds handling for OS-filtered empty test sets (warn for --changed, error otherwise).
Library/Homebrew/test/dev-cmd/tests_spec.rb	Adds coverage for brew tests behavior when OS filtering removes all runnable specs.

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