[llvm] [RISCV] Make XFAIL test UNSUPPORTED. (PR #168525)

[Mikhail Gudim via llvm-commits]

mgudim wrote:

I ran the test locally and it worked as I expected. I suspect it will be a lot of work to see why exactly it fails on some buildbots. Since this is only XFAIL temporarily, I hope it would be OK just to make this UNSUPPORTED.

https://github.com/llvm/llvm-project/pull/168525