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

[Min-Yih Hsu via llvm-commits]

mshockwave wrote:

Just had some discussions with Craig on naming and we're wondering: "no-no-xyz" sounds weird and so does "no-disable-xyz". Should we either give those features a better name or using "-" (and potentially "+" for positive features) instead of "no-"? This won't impact the core part of this PR, but just curious about folks' naming preferences.

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