Previously, the implicit warnings from force-bti (or gcs=always) weren't
possible to silence.
The force-ibt/cet-report flags could also be handled the same way, but I
haven't checked with GNU ld how they behave. And there, the force-ibt
flag only produces warnings if the IBT bit is missing, while cet-report
warns if either IBT or SHSTK are missing - but force-ibt probably
shouldn't implicitly start warning for missing SHSTK.
This addresses a discrepancy to GNU ld that was noted in #186173.
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