Export -latomic even if BUILD_TESTING is disabled. (backport #516)

[mergify]

Description

Replaces #384

Is this user-facing behavior change?

Did you use Generative AI?

Additional Information

---

This is an automatic backport of pull request #516 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 1879f2c has failed:

On branch mergify/bp/kilted/pr-516
Your branch is up to date with 'origin/kilted'.

You are currently cherry-picking commit 1879f2c.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   CMakeLists.txt

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[fujitatomoya]

Pulls: #517
Gist: https://gist.githubusercontent.com/fujitatomoya/76b83f8c1876d64b052322710fd2a40a/raw/04474327e4b5029f4f2b100a2ff85a2874a6d2e5/ros2.repos
BUILD args: --packages-above-and-dependencies rcutils
TEST args: --packages-above rcutils
ROS Distro: kilted
Job: ci_launcher
ci_launcher ran: https://ci.ros2.org/job/ci_launcher/17011

• Linux
• Linux-aarch64
• Linux-rhel
• Windows