Fix AVX2 simd support for ZEN2 AMD CPU.

[pkestene]

Enable AVX2 vectorization when KOKKOS_ARCH_AMD_AVX2 is ON for AMD Zen architectures.
Drop KOKKOS_ARCH_AMD_AVX2 macro in favor of KOKKOS_ARCH_AVX2.

[dalg24-jenkins]

Can one of the admins verify this patch?

[Rombur]

Is there a reason why we don't use KOKKOS_ARCH_AVX2 on AMD CPU?

[dalg24]

Is there a reason why we don't use KOKKOS_ARCH_AVX2 on AMD CPU?

We could not think of a good reason. We will drop KOKKOS_ARCH_AMD_AVX2 in favor of KOKKOS_ARCH_AVX2.
@Rombur agreed to implement these changes. Make sure you update the description accordingly and add to the changelog.

[dalg24]

Retest this please