<!--
Thanks for contributing a merge request!
We recommend that first-time contributors read our [contribution guidelines](https://eigen.tuxfamily.org/index.php?title=Contributing_to_Eigen).
Before submitting the MR, please complete the following checks:
- Create one PR per feature or bugfix,
- Run the test suite to verify your changes.
See our [test guidelines](https://eigen.tuxfamily.org/index.php?title=Tests).
- Add tests to cover the bug addressed or any new feature.
- Document new features. If it is a substantial change, add it to the [Changelog](https://gitlab.com/libeigen/eigen/-/blob/master/CHANGELOG.md).
- Leave the following box checked when submitting: `Allow commits from members who can merge to the target branch`.
This allows us to rebase and merge your change.
Note that we are a team of volunteers; we appreciate your patience during the review process.
-->
### Description
<!--Please explain your changes.-->
Broken at head by !2063.
See merge request libeigen/eigen!2066
<!--
Thanks for contributing a merge request!
We recommend that first-time contributors read our [contribution guidelines](https://eigen.tuxfamily.org/index.php?title=Contributing_to_Eigen).
Before submitting the MR, please complete the following checks:
- Create one PR per feature or bugfix,
- Run the test suite to verify your changes.
See our [test guidelines](https://eigen.tuxfamily.org/index.php?title=Tests).
- Add tests to cover the bug addressed or any new feature.
- Document new features. If it is a substantial change, add it to the [Changelog](https://gitlab.com/libeigen/eigen/-/blob/master/CHANGELOG.md).
- Leave the following box checked when submitting: `Allow commits from members who can merge to the target branch`.
This allows us to rebase and merge your change.
Note that we are a team of volunteers; we appreciate your patience during the review process.
-->
### 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
