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

[Yeting Kuo via llvm-commits]

yetingk wrote:

> Also, does this obey -menable-experimental-extensions?

IMO, I prefer this feature depends on `-menable-experimental-extensions`.  

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