[clang] [llvm] [RISCV] Update TargetAttr target-cpu override rule (PR #75804)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

It doesn't seem like a good idea to make target-independent logic behave in target-specific ways; that's going to confusing for both people hacking on clang, and for users if it's user-visible.  Is there some way we can make this logic consistent across targets? 

Maybe we can explicitly translate the "arch" feature into enabling/disabling all the relevant features?

https://github.com/llvm/llvm-project/pull/75804