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

[Craig Topper via llvm-commits]

topperc wrote:

> * The parser has to ignore unknown tuning features already for compatibility with other toolchains

Are we warning for unknown options from other toolchains or just silently ignoring?

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