[llvm] [RISCV] Introduce a new syntax for processor-specific tuning feature strings (Take 2) (PR #175063)

[Pengcheng Wang via llvm-commits]

https://github.com/wangpc-pp commented:

Is there a way to list all tunable features when using `-mtune=help` or something like that?

https://github.com/llvm/llvm-project/pull/175063