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### Description
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Enable generic clang backend tests.
Added an AVX512 job using the generic clang backend.
Also fixed up some guards in the custom AVX512 gemm/trsm kernels so they don't
start defining things if they aren't used.
See merge request libeigen/eigen!2063
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### Reference issue
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### What does this implement/fix?
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Currently, we run each test 3 times to account for flaky tests. Sometimes, the test fails so quickly that the random seed is the same for the subsequent test, which fails the exact same way.
This MR uses a nanosecond seed which resolves the issue described above. Now, if the test does not pass on the first attempt but passes on the retries, the gitlab job status will be yellow but still be treated as a pass in the ci/cd pipeline. Hopefully, this means we will get more passes and help us identify room for improvement.
### Additional information
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See merge request libeigen/eigen!2025
