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

[Yeting Kuo via cfe-commits]

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@@ -0,0 +1,7 @@
+// RUN: %clang --target=riscv64 -menable-experimental-extensions -c -o /dev/null %s
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yetingk wrote:

Yes. I am sorry that I didn't find out the obvious mistake. I am going to review my development process.

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