libultrahdr: fix build on i686-linux

[geri1701]

Link with libatomic on i686-linux to resolve the missing GCC atomic builtin reference.

The i686 build then reaches the test suite. Three i686-only rounding-sensitive
GainMapMath tests are filtered due to small floating-point differences.

Closes #532626

Assisted-by: pi coding agent / Mika (OpenAI GPT-5.5)

Things done

• Built on platform:
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• Tested, as applicable:
  • [NixOS tests] in [nixos/tests].
  • [Package tests] at passthru.tests.
  • Tests in [lib/tests] or [pkgs/test] for functions and "core" functionality.
• Ran nixpkgs-review on this PR. See [nixpkgs-review usage].
• Tested basic functionality of all binary files, usually in ./result/bin/.
• Nixpkgs Release Notes
  • Package update: when the change is major or breaking.
• NixOS Release Notes
  • Module addition: when adding a new NixOS module.
  • Module update: when the change is significant.
• Fits [CONTRIBUTING.md], [pkgs/README.md], [maintainers/README.md] and other
  READMEs.
• Follows the [automation/AI policy].

[yzx9]

Hi, is #533168 not working as expected?

cc @zwang20

[zwang20]

Before #533168, all x86 builds were reset to i386, which requires -latomic to compile.

[geri1701]

Thanks for the pointers. I missed #533168 in the issue timeline.

Since #533168 has already been merged into staging and is the more complete fix for
#532626, this PR is superseded. I’m closing it to avoid keeping a competing master-only
workaround around.