[X86] Use Skylake Server model for x86-64-v4 so we have full instruction coverage

The x86-64-v4 generic cpu arch supports AVX512BW/DQ/CD/VLX which isn't covered by the Haswell model, use the SkylakeServer model instead which is a lot closer to what the arch represents.

Differential Revision: https://reviews.llvm.org/D102553