Update Eigen to commit:3368ac6c6934af3d7c16b05727084b4e08d07497

CHANGELOG
=========
3368ac6c6 - Don't set platform-specific vectorization macros for generic backend.
fecfa7f27 - Fixes to make generic backend build with AVX512
ec93a6d09 - Add a generic Eigen backend based on clang vector extensions

PiperOrigin-RevId: 829167450
Change-Id: I074af9be5dd099ec9109e8d36964c3274ba5d234