[clang] [llvm] [RISCV] Teach .option arch to support experimental extensions. (PR #89727)

[Yeting Kuo via cfe-commits]

yetingk wrote:

@topperc  Sorry, there was a quick fix after the approve. `getTargetFeatureForExtension` could approve the extension with version. It may be better to allow them, but I am not sure the error message could be enough readable if we use wrong version number and is the format of output needed version number? 

https://github.com/llvm/llvm-project/pull/89727