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

[Sam Elliott via llvm-commits]

https://github.com/lenary approved this pull request.

Approving so this can fix the bots.

I presume the issue is some bots have asserts enabled and some don't

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