gh-129223: Raise KeyError in search_map_for_section() if not found

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• Issue: The compiler may optimise away globals with debug offsets #129223

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@pablogsal @ambv @encukou: This change fix the SystemError in test_external_inspection on Fedora Stable with LTO.

[ambv]

I'm not crazy about this approach because silently skipping tests when we couldn't find a section on a platform that we expected to be able to find the section is a bug. The fix in #129225 actually addresses the problem, which was the compiler optimizing away debug sections unless __attribute__((used)) was used.

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I can modify my PR to not skip the tests on KeyError.

[pablogsal]

I can modify my PR to not skip the tests on KeyError.

This should be fixed by the real fix #129225. Please, let's remove the skips because that will be masking actual errors that we want to fix. Let's merge the PR with the new raising of KeyError 👍

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@ambv @pablogsal: Ok, I modified my PR only to fix the SystemError, tests are no longer skipped.