Run CI across gcc/clang and Kokkos Serial/OpenMP, cache Kokkos

[Maccchiatooo]

No description provided.

[Maccchiatooo]

Closing for now: the gcc/clang split does not add libc++ coverage (Ubuntu clang links libstdc++, same as all three machine toolchains), and the OpenMP dimension cannot catch missing fences because Kokkos host dispatch is synchronous. Higher-value additions would be a sanitizer job and a macOS/libc++ job.

Note for whoever reopens this: adding the matrix renames the CI jobs, so the required status check 'Kokkos Serial + Open MPI' would stop reporting and block every PR. Add a fixed-name aggregator job first and switch the required check to it.