  Octo-Tiger, a large-scale 3D AMR code for the merger of stars, uses a
combination of HPX, Kokkos and explicit SIMD types, aiming to achieve
performance-portability for a broad range of heterogeneous hardware. However,
on A64FX CPUs, we encountered several missing pieces, hindering performance by
causing problems with the SIMD vectorization. Therefore, we add
std::experimental::simd as an option to use in Octo-Tiger's Kokkos kernels
alongside Kokkos SIMD, and further add a new SVE (Scalable Vector Extensions)
SIMD backend. Additionally, we amend missing SIMD implementations in the Kokkos
kernels within Octo-Tiger's hydro solver. We test our changes by running
Octo-Tiger on three different CPUs: An A64FX, an Intel Icelake and an AMD EPYC
CPU, evaluating SIMD speedup and node-level performance. We get a good SIMD
speedup on the A64FX CPU, as well as noticeable speedups on the other two CPU
platforms. However, we also experience a scaling issue on the EPYC CPU.
