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

[Sam Elliott via llvm-commits]

lenary 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?

I believe we are ignoring them entirely.

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