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

[Yeting Kuo via llvm-commits]

yetingk wrote:

> Does this disable use of experimental extensions for a `.option arch` in an inline assembly block without -menable-experimental-extensions.

No. my patch could not block the case. I will try to think this problem after my sleep.

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