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### Description
<!--Please explain your changes.-->
Broken at head by !2063.
See merge request libeigen/eigen!2066
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### Description
<!--Please explain your changes.-->
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
The goal of this MR is to implement a generic SIMD backend (packet ops) for Eigen that uses clang vector extensions instead of platform-dependent intrinsics. Ideally, this should make it possible to build Eigen and achieve reasonable speed on any platform that has a recent clang compiler, without having to write any inline assembly or intrinsics.
Caveats:
* The current implementation is a proof of concept and supports vectorization for float, double, int32_t, and int64_t using fixed-size 512-bit vectors (a somewhat arbitrary choice). I have not done much to tune this for speed yet.
* For now, there is no way to enable this other than setting -DEIGEN_VECTORIZE_GENERIC on the command line.
* This only compiles with newer versions of clang. I have tested that it compiles and all tests pass with clang 19.1.7.
https://clang.llvm.org/docs/LanguageExtensions.html#vectors-and-extended-vectorsCloses#2998 and #2997
See merge request libeigen/eigen!2051
Co-authored-by: Rasmus Munk Larsen <rmlarsen@google.com>
Co-authored-by: Antonio Sánchez <cantonios@google.com>
