[llvm] [RISCV] Adopt SpacemitX60's scheduling model for `-mtune=generic` (PR #167008)

[Paul Walmsley via llvm-commits]

paul-walmsley-sifive wrote:

I'd think that the most interesting test platforms for this patch would have in-order cores (like the Hazard3) , rather than out-of-order cores like the SiFive P550.  So from that point of view, it would be useful to know if there are significant regressions on other, common, in-order cores.

https://github.com/llvm/llvm-project/pull/167008