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

[via cfe-commits]

dyung wrote:

@yetingk, the two tests you added are failing on a couple of build bots:
- https://lab.llvm.org/buildbot/#/builders/164/builds/51995
- https://lab.llvm.org/buildbot/#/builders/258/builds/17867
- https://lab.llvm.org/buildbot/#/builders/91/builds/25943
- https://lab.llvm.org/buildbot/#/builders/280/builds/3159
- https://lab.llvm.org/buildbot/#/builders/283/builds/1636
- https://lab.llvm.org/buildbot/#/builders/139/builds/64910
- https://lab.llvm.org/buildbot/#/builders/259/builds/3993
- https://lab.llvm.org/buildbot/#/builders/285/builds/1756

Can you take a look and revert if you need time to investigate?

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