[llvm] [RISCV] Introduce a new tune feature string syntax and its parser (PR #168160)

[Craig Topper via llvm-commits]

topperc wrote:

> Another thing that came out this discussion, and related to [#168160 (comment)](https://github.com/llvm/llvm-project/pull/168160#discussion_r2548737453) as well, is that we probably want to have a curated list of accepted tuning directive for each CPU. For example, right now `single-element-vec-fp64` is only used by SiFive7's model, so for other CPUs we can say something like "single-element-vec-fp64 has no effect on the scheduling model" (or the other way around, only states the accepted tuning directives).

The list should be enforced by the compiler not by documentation.

https://github.com/llvm/llvm-project/pull/168160