guix: add -Wl,--icf=safe to darwin build #35197

--icf=safe Enable safe identical code folding.

This reduces the size of bitcoind by ~0.5mb, and bitcoin-qt by 1.3mb (aarch64-apple-darwin).

This option exists for lld, which is why it's only added for darwin builds.

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/35197.

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Reviews

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🚧 At least one of the CI tasks failed. _{Task i686, no IPC: https://github.com/bitcoin/bitcoin/actions/runs/25275669000/job/74105140405} _{LLM reason (✨ experimental): CI failed because the sock_tests test in the “Bitcoin Core Test Suite” failed (ctest reported 1 failing test out of 296).}

<details><summary>Hints</summary>

Try to run the tests locally, according to the documentation. However, a CI failure may still happen due to a number of reasons, for example:

• Possibly due to a silent merge conflict (the changes in this pull request being incompatible with the current code in the target branch). If so, make sure to rebase on the latest commit of the target branch.

• A sanitizer issue, which can only be found by compiling with the sanitizer and running the affected test.

• An intermittent issue.

Leave a comment here, if you need help tracking down a confusing failure.

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